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!