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
The HTTP nerd in me wonders if this method should use a HEAD request, since we only really care about the distinction between a 200 and a 404. Might be overkill, though.