Add support for Yo::Gabba::Gabba

This addresses https://github.com/hybridgroup/gabba/issues/24.

Can either do it this way, as an alias, or restructure so that `Yo` is the actual root module and `Yo::Gabba::Gabba` is the preferred way to create a new instance.

Example code:

```ruby
@gabba = Yo::Gabba::Gabba.new 'abc', '123'
#=> #<Gabba::Gabba:0x007f8efb282468 [...]>
```