Rename PrivilegeSet to Privilege

A Privilege is now a name for a set of controller actions.
Update GenericRole fixtures and use them
Take attrs instead of name in Privilege constructor
Add Arrthorizer::Rails::ControllerAction

This class will make it possible to easily find the Privilege that
should be checked for a certain request.
Do not provide Privileges to ControllerAction constructors

Instead, provide them via a separate accessor. The ControllerAction can
then be meaningfully constructed without having any linked privileges.
Boyscout: improve factoring of Arrthorizer::Rails::Configuration

It is now easier to a) insert a fake configuration when testing and b)
change the configuration file location. Refs #18