Initial release

In order to modularize and share our stack, we are releasing our `BackboneApiClient`. This is a boiled down version of what we had (e.g. removed built-in `_method's`) but that was part of the benefit of moving it under a modular lens.

README and code can be viewed at: https://github.com/uber/backbone-api-client/tree/dev/inital.release.sqwished

In this PR:

- Added `mixinModel` and `mixinCollection` methods
- Added tests for affected model/collection methods
- Added documentation

Fixes #1 

/cc @mlmorg @orbiteleven @zheller