Add ActiveSupport::Notification instruments

I've started adding support for ActiveSupport::Notification instruments to devise. Before I get too carried away, I'd like to know if this approach is satisfactory, and if there are any specific places that should emit notifications.

Also, test_unit isn't exactly my cup of tea, so I could use some pointers in how to test these instruments, if that's necessary.

This is still a work in progress.
I think we should name this module `Instrumentation`. `Notifiable` might sound like one of the existing modules that add the full stack behavior, like `Rememberable` or `Confirmable`.