Puts top level Boolean class in module HappyMapper

Putting classes in the top level namespace leads to havoc in the case when other libraries/gems use a Boolean class (for example [virtus](https://github.com/solnic/virtus)).

The seemingly strange `HappyMapper::Boolean` in the spec is not so odd really. The main thing is that all the references to Boolean in `happymapper_spec.rb` work.