Adds ponyd LaunchAgent installer script

I added a Python script that will install a LaunchAgent plist file in `~/Library/LaunchAgents` on Mac OS X.

Every time your machine boots (and your user logs in), this will be executed:
`/usr/local/bin/ponyd serve --listen-interface=127.0.0.1`
...launching a PonyDebugger portal at [http://localhost:9000](http://localhost:9000).

Usage:
`$ scripts/install_ponyd_daemon.py`