Renamed GrapeEntity to Grape::Entity.

I am a little concerned moving `Grape::Entity` into a new namespace `GrapeEntity::Entity`, it will force people to update a lot of stuff. And I think there's no real good reason not to keep entities inside the `Grape` namespace.

There's a collision on `Grape::VERSION`, so I left that one alone as `GrapeEntity::VERSION`. 

I think I have fixed .gemspec with all the correct runtime dependencies, but we need to port a lot of the test code that lives behind Grape API in here, such as for `to_xml`.