feat($timeout): Add debouncing support to $timeout service

This feature allows resetting the timer on an ongoing `$timeout` promise.

A fourth optional argument is added to `$timeout` which is the
old promise to be reset.

I.e.: `promise = $timeout(fn, 2000, true, promise);`

This will call `fn()` 2 seconds after the last call to the `$timeout()`
function.