Add ability to send arbitrary UNIX signals to processes:

* Extend the Process class to handle non-killing signals
* Expose this via the XML-RPC API
* Add supervisorctl `signal` command

This is an update of PR #228 and hopefully moves the ball forward on #179, #53, etc.

Cc: @squeed, @weebly, @mnaberez, @sontek
Does this need to be done async?