Added dropwizard integration for config file support

Added dropwizard integration for metrics-librato to automatically add LibratoReporter via a dropwizard configuration file from dropwizard 0.7.0. You can see an example at https://github.com/dropwizard/dropwizard/blob/v0.7.0/docs/source/manual/configuration.rst#graphite-reporter

I added this functionality as a separate maven project. I wasn't sure if @librato wanted to add this functionality as a separate module in `metrics-librato`.