added jcollect network listener collector

This collector is able to receive metrics from one or more jcollectd instances. It has an UDP listener in a separate thread for this purpose. It does some try to extract the JVM instance name and make a nice metricname. (I hear you ask: "why are you using Diamond and not collectd?" Collectd's design does not allow for free metricnaming. You're stuck with 4 fields: plugin, plugininstance, type and typeinstance.)

Jcollectd can run as a java agent inside an JVM and gather metrics from mbeans. Normally jcollectd sends its data to a collectd server, but since the protocol is open and python code was available I made a Diamond collector.

Background info:
* https://collectd.org/wiki/index.php/Plugin:Network
* https://github.com/collectd/collectd/blob/master/contrib/collectd_network.py
* https://github.com/hyperic/jcollectd instances