Begin writing YARD documentation

* Install YARD
* Add a `rake documentation` task
* Make `endpoint` and `resource_name` methods private
* Write docs for `Configuration` and `RestfulModel`
I think that's a good rule. Trying to maintain this pattern is untenable, and it could very easily get out of sync.

As a sidenote, we should develop a little style guide for writing API documentation One Of These Days.™