Add support for namespaced models

This PR adds support for models inside a module.

e.g.

    module Foo
      class Bar
        include Cequel::Record

        belongs_to :moo, class_name: 'Foo::Moo'
      end
    end


This will create a column family named:  `foo__bars`.

I've used a double underscore as a separator to avoid conflicts with models named `FooBar`.

To use `belongs_to` and `has_many` you'll have to specify the `class_name`.