Improvements to split_otu_table.py

This PR reduces the code complexity of split_otu_table.py and adds functionality
to allow a user to pass multiple fields to split the biom table on. Adds significant documentation to the library code and the script.
addressed comments
remove unused imports
Merge branch 'master' into split_otu_table_update
Updated ChangeLog.md
Merge pull request #1663 from wdwvt1/split_otu_table_update

Improvements to split_otu_table.py
Add test for write_seqs_to_fastq
Change test to reflect new api

split_fasta_on_sample_ids_to_files got a few new arguments
Change write_seqs_to_fasta; use context manager
Add write_seqs_to_fastq function
Change name of function
Change name of function in tests
Add option for FASTQ files, usage example
Use new function API where needed throughout QIIME
Rename qiime_test_data dir and add fastq file
Add mention of changes to ChangeLog.md
Merge pull request #1675 from adamrp/split_fastq

Add FASTQ splitting by sample id to qiime.util