First version of JMXMonitor

At first I thought I could expand DbMon or Perfmon, but it made more sense to create an entirely new listener. 

The code is very much inspired by DbMon. 

Config should be pretty much self-explanatory.

This is my first pull request on github, so apologies if there is information missing or something. There may also be some DbMon-specific code that is no longer applicable in JMXMonitor.