Add scikit-bio 0.1.1-dev as dependency
Remove CategoryStats and Best from qiime.stats

Also remove formatting code in qiime.format. Still need to update code to use new bioenv function in skbio.
Update compare_categories to use skbio's bioenv

Still need to update docs to reference BIO-ENV instead of BEST, but tests are passing!
Rename BEST to BIO-ENV (and 'best' -> 'bioenv')
Minor update to comments in compare_categories.py
Merge pull request #1584 from ElBrogrammer/bioenv

Use scikit-bio's bioenv method in favor of qiime.stats implementation