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
```
supervisor> signal FOO dog:1 dog:2
error: <type 'exceptions.ValueError'>, Unknown result code 2 for dog:1: file: /Users/marca/dev/git-repos/supervisor/.tox/py27/lib/python2.7/site-packages/supervisor/supervisorctl.py line: 755
```