Strong Parameters (and some other improvements)

This is gonna hurt... This update changes from using `attr_accessible` to using [Strong Parameters](https://github.com/rails/strong_parameters) (sort of preemptively, since it will be default when we move to Rails 4).

All tests are passing, but given our test coverage is severely lacking, there will be broken features. I plan to go ahead and merge and fix (and write failing tests for) broken params as I find them.