add Active Record scope only if scope option is passed

Use scope option to specify Active Record scope name.
raise ArgumentError when enumerized attribute is already defined
extract Enumerize::Module class
build predicates on enumerize module
support AR scopes when attr is defined in a module
Merge pull request #55 from brainspec/module-scopes

support AR scopes when attr is defined in a module
Merge remote-tracking branch 'upstream/master'