add riemann package to report metrics to riemann

hi,

We use riemann and so have added a package to report events to there. The code can't be immediately pulled in without changing the import to reference rcrowley/go-metrics rather than pingles/go-metrics but that should be it.

Hope it's useful,
Paul @ uSwitch
Why not append the events to a slice and send them all with a `for` at the end much like the influxDB sink does? It would greatly reduce duplication.