Split libraries rework

**NOT READY FOR MERGE YET**

I apologize in advance for the size of this pull request, and for the size of the associated comment. 

This pull request covers an attempt at redoing the core concept behind split libraries (incl. fastq) with a lot of input from @walterst early on. This is not a complete rewrite of `split_libraries.py` or `split_libraries_fastq.py`, though it covers most of the latter. The intent was to handle some of the core and primary functionality, support fasta/fastq/whatever, develop an easy to extend and test framework, and then merge in additional support and options as we proceed forward, assuming the concept being used here makes sense as a replacement. At the end of this long description is an example of the command and the type of data that is yielded by the `Workflow`. The use of the `Workflow` object allows for:

* execution of methods can be done based on runtime options (e.g., barcode-type)
* execution of methods can be done based on runtime state (e.g., only execute qual code if there is qual data)
* grouping of like methods (e.g., all quality oriented methods are part of wf_quality)
* updating state as processing on a single item proceeds
* tracking of stats
	
If the concept does make sense, then there are a few areas that I'd like help with for guiding this to completion. Specifically:

* Many of the option names were more or less carried over. Do we want to retain these? 
* Currently quite a few stats are tracked by the `Workflow`, are there others that would be nice to have?
* The log file is not currently being written but most of the necessary data are there. Should this just write out in a similar to existing form, or do we want to use a Python or IPython logger?
* fastq writing is not supported yet and fasta writing is using a inline formatter. I didn't see a fasta formatter that could take a single sequence in skbio (0.1.4 i think is the qiime dep) 
* Actual 454 support. Primer handling is not yet exposed to the command line. There is an indirection in the Workflow for instrument specific checks that I wasn't sure about whether we wanted, which blocked actually exposing this.
* Some of the logic of `split_libraries_fastq.py` surrounds manipulating inputs (e.g., rc of mapping file barcodes) or manipulating outputs (e.g., rc of reads being written). These are transforms that are external to the `Workflow` or sequence iterators. Can some of this logic be managed elsewhere? 
* The use of the failure callback in `slib` when iterating the `Workflow` here is somewhat awkward, but that is currently the best way to handle sequences that fail.
	
A few deviations:

* Using `--sample_ids` is not currently easy to support as the `Workflow` does not know what file a given sequence originated from at this time. Is it critical to support this option? If so, it can be done external to the `Workflow`.
* `--retain_unassigned_reads` is just on all the time, much easier to do it this way. This currently includes all failed sequences as well. Is it necessary for the unassigned reads to be raw or should they be processed, and should this include failed reads (e.g., poor quality)?
* `--store_qual_scores` is done only if `--to-fastq` is used
* only a single mapping file can be passed in. This was done as there isn't a distinction between sequence and source file. Just like `--sample_ids`, this can be done external to the workflow if this is critical
	
In addition to the split libraries work, this PR also includes a new command line interface via click, and is stubbed out for a possible structure that can be used for other scripts. This is meant as a possible starting point for using click, and I'm very curious about what other developers think here. The primary point of execution is `scripts/qiime`. This file simply imports the click command group (`qiime_cli`) and any other click commands, and additionally, makes the qiime config readily available to any other command via `ctx`. Though the commands aren't used directly within `scripts/qiime`, the import makes them part of `qiime_cli`. The next piece is `qiime/cli.py`, which provides the qiime command group, and which we likely want to expand out as these are bits that will be common to qiime. Last, there is a `qiime/click_commands.py` script that contains the `slib` command. There were a few choices made here that should be explained:

* imports are done within the `slib` method. The benefit is that those imports are _only_ executed if the slib command is actually used. If they were at the script level, they would be imported on import of `slib` thereby adding overhead.
* `ctx` is passed in, which makes it trivial to access a common qiime config
* the `Workflow` takes `kwargs`, but not all of the `kwargs` are relevant for the workflow. As a result, the unused ones are popped off. 
* I didn't see a way to "group" click options on the command line. Right now, there are comments that denote some structure to the options but this is not reflected on the command line.
* There are various methods defined within `slib`, the motivation to minimize import overhead if `slib` is not actually executed.
	
Example output:

```bash
$ ./qiime
Usage: qiime [OPTIONS] COMMAND [ARGS]...

  QIIME, canonically pronounced 'chime'

Options:
  --version
  --help     Show this message and exit.

Commands:
  slib  Quality filter and demultiplex sequences
$ ./qiime slib --help
Usage: qiime slib [OPTIONS]

  Quality filter and demultiplex sequences

Options:
  -i, --sequence-read-fp PATH     Input sequence reads  [required]
  -o, --output-dir PATH           [required]
  -m, --mapping_fp FILENAME       Mapping file  [required]
  -b, --barcode-read-fp PATH      Barcode read files
  --rev-comp / --no-rev-comp      Reverse complement sequences on output
  --start-seq-id INTEGER          The starting unique ID for sequences
  --to-fastq                      Write out in fastq
  --rev-comp-barcodes / --no-rev-comp-barcodes
                                  Reverse complement barcode reads
  --phred-offset [33|64]          The ASCII offset used to decode PHRED scores
  -q, --phred-quality-threshold INTEGER
                                  Minimum PHRED quality score
  --barcode-type [golay_12|hamming_8|not-barcoded]
                                  The type of barcode used
  --max-barcode-error FLOAT       The maximum number of barcode errors allowed
  --retain-primer / --no-retain-primer
                                  Whether to retain the primers or not (if
                                  applicable)
  --max-primer-mismatch INTEGER   Maximum mismatches allowed within the
                                  primers
  --min-seq-len INTEGER           The minimum sequence length
  --max-ambig-count INTEGER       Maximum ambiguous bases allowed
  --rev-comp-mapping-barcodes / --no-rev-comp-mapping-barcodes
                                  Reverse complement the mapping barcodes
  --help                          Show this message and exit.

# Notice that multiple input files and barcodes can specified without using commas
$ ./qiime slib -i ../qiime_test_data/split_libraries_fastq/lane1_read1.fastq.gz -i ../qiime_test_data/split_libraries_fastq/lane2_read1.fastq.gz -m ../qiime_test_data/split_libraries_fastq/map.txt --barcode-type=golay_12 -b ../qiime_test_data/split_libraries_fastq/lane1_barcode.fastq.gz -b ../qiime_test_data/split_libraries_fastq/lane2_barcode.fastq.gz --rev-comp-mapping-barcodes -o slibtest
```

And finally, here is what one of the yielded items from the Workflow looks like (pprint'd) and the resulting stats tracked for processing a the test data:

```python
# resulting state for a sequence that was successfully processed
{'Barcode': 'AGAGTAGCTAAG',
 'Barcode Qual': None,
 'Barcode errors': 0,
 'BarcodeID': 'M00176:17:000000000-A0CNA:1:1:18006:1821 1:N:0:0',
 'BarcodeQual': array([28, 33, 25, 32, 35, 35, 37, 37, 39, 39, 38, 35], dtype=int8),
 'BarcodeQualID': 'M00176:17:000000000-A0CNA:1:1:18006:1821 1:N:0:0',
 'Final barcode': 'AGAGTAGCTAAG',
 'Forward primer': None,
 'Original barcode': 'AGAGTAGCTAAG',
 'Qual': array([33, 31, 31, 37, 37, 37, 37, 37, 39, 39, 38, 39, 39, 41, 41, 41, 41,
       41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 40, 41, 41, 41,
       40, 41, 38, 38, 39, 40, 41, 41, 41, 41, 41, 40, 40, 41, 39, 38, 40,
       39, 27, 32, 36, 33, 35, 12, 22, 27, 30, 33, 30, 29, 25, 25, 34, 34,
       34, 35, 25, 31, 34, 18, 29, 34, 35, 31, 32, 27, 25, 31, 34, 31, 29,
       31, 34, 25, 29, 34, 34, 34, 33, 34, 34, 31, 35, 35, 35, 35, 19, 25,
       29,  7, 10, 19, 24, 24, 29,  7, 17, 30, 33, 29, 33, 27, 33, 24, 18,
       18, 25, 25, 25, 29, 29, 34, 34, 32, 25, 19, 29, 16, 15, 20,  8, 15,
       25, 19, 10, 25,  7,  7, 10, 25, 32, 32,  7,  7, 19, 29, 34], dtype=int8),
 'QualID': 'M00176:17:000000000-A0CNA:1:1:18006:1821 1:N:0:0',
 'Reverse primer': None,
 'Sample': 's173',
 'Sequence': 'AACATAGGTCACGAGCATTATCCGGATTTATTGGGCGTAAAGGAAGCGTAGGCGGGGAGGTTGATCCATTGTTAAAAGCATTTGCTTAACAAATGTGTGCATTGGAGATCGCCTCCCTAGAGTTAATCAGGGGGTACTGGAATTCAATGTG',
 'SequenceID': 'M00176:17:000000000-A0CNA:1:1:18006:1821 1:N:0:0'}

# resulting stats from processing split_libraries_fastq test data (lane1 and 2)
{'barcode_corrected': 262,
 'exceed_barcode_error': 252,
 'exceeds_max_primer_mismatch': 0,
 'index_ambig_count': 4,
 'max_ambig_count': 2,
 'min_per_read_length_fraction': 0,
 'min_seq_len': 0,
 'quality_max_bad_run_length': 0,
 'sample_counts': Counter({'s41': 8, 's173': 6, 's72': 6, 's120': 6, 's121': 6, 's32': 6, 's141': 6, 's53': 4, 's51': 4, 's101': 4, 's183': 4, 's189': 4, 's17': 4, 's99': 4, 's124': 4, 's133': 4, 's132': 4, 's167': 4, 's179': 2, 's178': 2, 's172': 2, 's174': 2, 's177': 2, 's176': 2, 's55': 2, 's59': 2, 's58': 2, 's135': 2, 's109': 2, 's105': 2, 's107': 2, 's102': 2, 's184': 2, 's185': 2, 's186': 2, 's83': 2, 's181': 2, 's76': 2, 's188': 2, 's87': 2, 's44': 2, 's45': 2, 's117': 2, 's115': 2, 's111': 2, 's110': 2, 's47': 2, 's191': 2, 's70': 2, 's75': 2, 's74': 2, 's77': 2, 's90': 2, 's97': 2, 's98': 2, 's128': 2, 's129': 2, 's122': 2, 's126': 2, 's127': 2, 's68': 2, 's66': 2, 's64': 2, 's62': 2, 's63': 2, 's80': 2, 's81': 2, 's136': 2, 's130': 2, 's88': 2, 's139': 2, 's38': 2, 's35': 2, 's30': 2, 's33': 2, 's140': 2, 's142': 2, 's145': 2, 's146': 2, 's148': 2, 's149': 2, 's151': 2, 's150': 2, 's157': 2, 's154': 2, 's159': 2, 's2': 2, 's7': 2, 's169': 2, 's166': 2, 's165': 2, 's161': 2, 's22': 2, 's26': 2, 's27': 2}),
 'sequence_count': 500,
 'sequence_lengths': Counter({151: 242}),
 'unknown_barcode': 234,
 'unknown_primer_barcode_pair': 0}
```