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
I'm a little nervous about all of these private class methods. Would it make sense to bring back some kind of `Namely::Client` object to handle the requesting API calls and JSON conversion?