Change to block parameters allows the user of this library to access the association.  The to_obj and from_obj block parameters were completely unnecessary as the user is calling the library with those parameters and thus already knows them.
Rename parameter :only to :types; also allow symbols to be passed as well as strings
Convert specs to Ruby 1.9 Hash syntax
Small refactoring, remove uncessary code
Changed API slightly to allow :only => [Class] and :except => [Class]