resolves issue #1649

adds documentation to what file format is acceptable for the script when using t
he --sample_id_fp option.

cleans up the lines of code that do the filtration when that option is passed.
Fixing qiime/rarefaction.py
Fixing qiime/split.py
Merge pull request #1652 from josenavas/biom21

Biom21
API/ENH: QIIME now depends on BIOM 2.1

QIIME should now depend on BIOM 2.1 and not in BIOM 2.0.1-dev, a note
has been added to the ChangeLog.md file.

Fixes #1590
Merge pull request #1654 from ElDeveloper/biom-2.1

API/ENH: QIIME now depends on BIOM 2.1
Merge pull request #1650 from wdwvt1/issue_1649

resolves issue #1649