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 conflicted about this: I'm reluctant to add a dependency on ActiveSupport, but defining `self.pluralize` in every model makes me sad.