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 you need to use [`private_class_method`](http://ruby-doc.org/core-2.0.0/Module.html#method-i-private_class_method) here, unless something's changed in a recent Ruby version (although, even if it has, we might want to support older Rubies).