Current results

This branch needs merging so that topic branches can be used from now on. It contains many loosely related changes, such as:

* Make Arrthorizer self-loading via ```autoload``` tree.
* Allow default Context to be configured per Rails Controller
* Allow action-specific Context to be configured per Rails Controller
* Add #merge to Context, allowing defaults to be reused in action-specific contexts.
* Allow Rails controllers to build Contexts at runtime
* Add some documentation
* Add ```Permission.grant```
* Make ```rake``` run the spec suite
* Add Travis integration
* Add ArrthorizerException which nests rescued Exceptions automatically