add pushover.net notifications.  copied growl and modified.

Hello,

I thought it'd be nice to have a third party service for notifications other than growl.  pushover.net is a service where you pay for the app once ( android/ios ), and then all push messages are free.

I'd like to generalize the notification system a little bit so other notification services are easier to add, but, I thought I'd just start with something small.

-corris