Dispatch an event in arc land right before pushing a revision

Summary: I looked at the pros & cons at adding hooks in git/hg vs arc land and I prefer arc land.

Test Plan:
* added an event listener and made sure I could handle the event.
* made sure things get reverted when the event handler throws an exception.

Reviewers: vrana, epriestley, pieter

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5268