Checkpointing sketch for BigDataFrame
Initial cut of some BigDataFrame features (untested)

Includes the BDF object along with projection and row selection (i.e., pandas indexing)

Includes some SQL AST mostly modeled after Impala's front end SQL analyzer.

Also includes a bunch of commented out code from earlier sketches.  I just want to have these in the repo, and will shortly delete them.
Some cleanup
Mostly work on creating BDFs
Mainly added ImpalaContext and rudimentary join functionality
Added a few more pandas functions; started get_dummies
Nominally working BDF including GroupBy
Cleaned up clean-up code and added .count() and .collect()
Post-rebase fixes