Add Ability to use Structs as data objects

This adds auto detection of Structs via the #members method, similiar
to how columns are auto detected from hashes.
Force Struct members to symbols.

Ruby 1.8 structs return members as strings while 1.9 structs return symbols.
Merge pull request #44 from aselder/struct_data

Add Ability to use Structs as data objects