comprehensive compute tests
mongo: distinct
add pymongo to comprehensive compute test
a few additions to compute test_comprehensive
Merge branch 'master' into compute-comprehensive
DataFrame -> pd.DataFrame in into.py
compute: only add mongo source if pymongo exists
try-except mongo imports
compute: avoid bcolz testing issue

This bcolz issue https://github.com/Blosc/bcolz/issues/37 was
intermittently causing errors in tests after selections were used.

We can avoid this issue by having pretty lame selection tests.
Merge pull request #478 from mrocklin/compute-comprehensive

comprehensive compute tests