Build an event dispatch mechanism into Phabricator

Summary:
This is an attempt to satisfy a lot of the one-off requests a little more
generally, by providing a relatively generic piece of event architecture.

Allow the registation of event listeners which can react to various application
events (currently, task editing).

I'll doc this a bit better but I wanted to see if anyone had massive objections
to doing this or the broad approach. The specific problem I want to address is
that one client wants to do a bunch of routing for tasks via email, so it's
either build a hook, or have them override most of ManiphestReplyHandler, or
something slightly more general like this.

Test Plan: Wrote a silly listener that adds "Quack!" to a task every time it is
edited and edited some tasks. I was justly rewarded.

Reviewers: nh, jungejason, tuomaspelkonen, aran

Reviewed By: aran

CC: aran, epriestley

Differential Revision: 881