Add `find` and `all` model methods

This adds a `find` and `all` method to all MR models. These use
their matching methods on an ActiveRecord object, but properly
wrap the returned records in an instance of the model. These are
mostly intended for convenience, but generally specific finder
methods is encouraged over using these.

Closes #7
Merge pull request #16 from redding/jcr-add-find-and-all

Add `find` and `all` model methods