Support RiemannReporter

This adds support for the riemann reporter included in the riemann java
client.

Props to Aaron Morton who wrote about his verison of this addition in
his blog:
http://thelastpickle.com/blog/2013/11/22/4-hours-with-riemann.html but
forgot to push the code to github!
Remove defaults for optional RiemannReporter options

Discovered a bug where the `service` field of the metric emitted by the
`RiemannReporter` was prepended by a space. Turns out this was caused by
setting prefix to an empty string.

To prevent any similar bugs from occurring I removed all default values
from the `RiemannReporterConfig`.

Setting the optional prefix setting on the RiemannReporter caused th
use a more recent version of riemann-java-client
Do not pull in metrics from deps

metrics-reporter-config should be in charge of it's own metrics version
remove unused import