Support for compound keys!!1!

- Added support for compound keys in
  ResultSet.read_attribute_for_validations
- Changed the internal data structure for ResultSet to retain some of
  it's structure
- Updated code for flattening rows into ResultSet
- Updated Validation test to use new compound key for group validation
Fixes #7.  Added a test for string keys
Removed pry require
Merge pull request #8 from liftopia/composite_keys

Looks good to me!
Merge remote-tracking branch 'origin/master' into resultset-rollup
Altering embiggen code for new ResultSet structure

The structure of the ResultSet objects changed internally to
support duplicate named keys so the code to embiggen (the opposite
of flatten) the result sets needed a corresponding update.