Adding getRange operation, adds support for #8
Fixes for getRange.

Fix thrift code to correctly distinguish between null and all
other falsey values in KeyRange object.

Make start/end defaults match the way pycassa does it. I was unable
to get this to work with the start_token/end_token defaults.

Added test for fetching out all rows from a cf.
getRange: add flag to limit number of rows.
Merge remote-tracking branch 'refs/remotes/origin/get-range'