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.
`payload` should be a Hash, and we should merge it with the `default_instrument_payload` Hash. This way we would avoid calling the `default_instrument_payload` method in the `send_confirmation_instructions.confirmable.devise` instrumentation.