Activerecord 4

This patch improves compatibility with ActiveRecord 4 and does a great deal of cleanup.  The changes include the following:

* [Removal of Squeel](http://erniemiller.org/2013/11/17/anyone-interested-in-activerecord-hackery/).
* Updated deprecated/removed methods such as `#scoped` -> `#all`.
* `require 'test/unit'` -> `require 'minitest/autorun'`
* Prefer class methods over scopes.
* General cleanup.

@neildecapia @flippyhead Please review!