allow for models inside a module

For some reason ruby doesn't like modularized constants specified as symbols.

    > module Foo; class Bar; end; end
    > Object.const_get(:'Foo::Bar')
    => NameError: wrong constant name Foo::Bar
    > Object.const_get('Foo::Bar')
    => Foo::Bar

This commit fixes that, allowing for modularized models in associations.