Watchdog plugin (circus sprint day 2)

This plugin waits for udp messages (sending simple text lines, parsed via regex).
If no update is sent from the watched process, the plugin will force a kill for this specific process, letting circus restart it automatically.

The purpose of this plugin is to handle some cases of 'frozen' processes that seems to work, but does not work anymore, despite some cpu load and/or memory consumption.

Of course, it is needed to write some code in the watched programs to be able to send the heartbeat messages.