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.
expr: Summary
expr: summary hashable and eval(str(.)) able
python: Summary compute
scalar: name, dtype properties
expr: Reductions support Number interface
compute: python arithmetic
python: summary by
pandas: summary by tests
pandas: summary by
pandas: summary
sql: summary and by summary
expr: name attribute for TableExpr

in case a TableExpr is a column
spark: summary
don't test base spark

Sadly we can have only one SparkContext