Modified compare_alpha_diversity.py

This commit modifies compare_alpha_diversity.py so that it will output the average alpha diversity of the compared categories. I find myself wanting this information frequently when doing tests and it wasn't available. These changes modify the output of the script so that you pass a directory to be created and it puts two files in that directory. One specifying the comparisons and their values, and one specifying the averages and std's of the averages for each treatment group. I have updated the script usage tests and verified that they work so that jenkins should be happy.

In addition, I needed to modify core_diversity_analysis.py because it depended on the output. I only had to modify several lines in the workflow/core_diversity_analysis.py code to alter the way it was handling the output directories.
Removed old test files which have been migrated.
Modified tests

Tests now use assertFloatEqual where appropriate which will hopefully resolve the failures with the latest jenkins build for compare_alpha_diversity.py. Also changed some locations in test_workflow/test_core_diversity_analysis.py to reflect changes in output structure of compare_alpha_diversity.py
Modifies test_downstream.py to account for alpha div changes.
Modifies to address comments.
Forgot a documentation change.
Modified documentation in script and Changelog.md
Commiting modified test_downstream.py

Avoids automerge conflict resulting from simultaneous edits.
Merge branch 'master' of https://github.com/qiime/qiime into compare_alpha_div_improvement

Conflicts:
	tests/test_workflow/test_downstream.py
Fixing illumina quality chars to return only printable chars
Added usearch61 chimera checking (denovo and ref) to identify_chimeric_seqs.py and associated unit tests.  Two related functions added to pycogent_backports/usearch.py along with associated unit tests.  Slight modification to qiime/util.py for fasta splitting function on SampleIDs to return a list of output filepaths.
Added tutorial section for usearch61 chimera checking
Added comment to changelog regarding tutorial addition for identifying chimeric sequences and usearch61
Fixed conflict in scripts/identify_chimeric_seqs.py (authorship list)
Fixed conflict with authorship list in scripts/identify_chimeric_seqs.py
added template document
added several sections
added several more sections
Modified usearch61 denovo chimera check to just look for Y and N chimera flags, as some system-dependent randomness affects other numbers for uchime output file.
Modified identify_chimeric_seqs.py script to have single credits line with all authors, modified script example with usearch61 to not use split_by_sample option to avoid error when output file already exists.