Add metrics reporter for riemann

This adds a metrics reporter that submits data to Riemann. Requires riemann-java-client 0.2.9, which I'll see if @aphyr will release today.

I'd much rather use something more abstract like [metrics-reporter-config](https://github.com/addthis/metrics-reporter-config) but it doesn't support codahale metrics 3 or riemann. Maybe later.