Added hRaven integration.

Adding HRavenStatsWriteService , that can be used with any of the existing "notifiers" to send statistics over to hRaven. 
For now, I only "plugged" it into the cascading/scalding modules, but the implementation is generic enough, so that integrating with Pig (and hive) should be trivial. So, I put the service itself into ambrose-common to be available for future integrations (which may, probably, be able to benefit form the AmbroseCascadingNotifierFactory implementation I also added, which can be generalized and moved to common at that point as well).