Update hoptoad=>aibrake in rake task example
Updated Heroku README to use Airbrake's new name.
* The instructions for Rails 1 and 2 are untested by me.
* A few places still use "Hoptoad" and the README reflects that.
* I rewrote "configure your notification settings" to reflect
the UI as I just experienced it.
Add Heroku deploy notification
Add instructions for heroku gem to Heroku addon README
Adding Heroku notifier readme