Add CompositeGraph, ActiveThreadsOverTime and PerfmonCollector support

This PR adds support for CompositeGraph, ActiveThreadsOverTime and PerfmonCollector that all are a part of the jmeter plugins, hence the comment in the example.

Since I'm really no Ruby nor jmeter master, please let me know if I can do anything in a better way.

Also added example for PerfMon and link to instructions for setup.