Refactor to ActiveRecord-style models

* Inherit models (like Profile) from a RestfulModel
* RestfulModel implements .all, .find, .exists?
* Scrap the idea of Client objects
* Implement a `rake console` task for easier manual fiddling
Use HEAD (not GET) to test if a resource exists
Pull network interaction into a Client object

* Extract private class methods from RestfulModel
* RestfulModel doesn't have to know about network interaction
* Can now test JSON responses! (some duplication currently)