Minitest and avoid adding methods to the model

This does two things : 

* Add MiniTest and tests to check encryption and decryption work as expected.
* Refactor the module to avoid adding methods to the model.

Previously, the encrypt and decrypt methods were added to all ActiveRecord models.
As these names are quite common, they might conflict with other method names in the models.
This uses an other class so the only method added to the models is attr_secure.

I also removed the dependency to ActiveSupport since it was a trivial change and avoids loading an other framework.