Added intervalEvent attribute and an example

Added intervalEvent which broadcasts "event:timer-interval" on the
$rootScope every interval.

This allows periodic events that can be consumed, for instance, to save
a document occasionally.
Updated description with fork changes
Generalized the timer intervalEvent

Allows a unique event to be broadcast on the $rootScope from each timer
so that you can have events fired based on different intervals.
Merge pull request #1 from timothyleerussell/master

Awesome... I like this change. I will make minor cleanup before pushing ito to gh-pages