Use SciKit-Bio in qiime/stats.py

With these changes we replace combinate with itertools.combinations,
additionally some imports from cogent have been replaced with scikit-bio
imports.
Fix non-pythonic iterations
Merge pull request #1436 from ElDeveloper/remove-combinate

Use SciKit-Bio in qiime/stats.py
Merge https://github.com/qiime/qiime into removing_util
this should pass all tests