Added a plugin for Resmon

Hi,

We've created a quick plugin that can pull metrics from [Resmon](http://labs.omniti.com/labs/resmon) and push them into Riemann.

Resmon can collect many interesting metrics and has a few plugins itself.

I guess the primary usage for the riemann-resmon plugin would be for people already running Resmon, with just one line for localhost in the hosts file. We however will prefer to have the riemann-resmon just running on one machine, polling multiple Resmons over HTTP. The plugin supports both scenarios.