Initial commit of model building

This gives us a chance to annotate the models in whatever way
we need.  I've added support for:

* Adding new top level keys.
* Updating anything in the 'operations' dict with new attributes
  (param annotations, operation annotations, etc).
* Merging in the data from _services.json into the 'metadata' top
  level key.

Things I've left out for the time being:

* Annotating snake case conversions for ops/params.
* Adding python types to the model types.
Regen models using new build process