refactoring build and tests

You can now override both the keyspace and node address via system
properties. Common test set up and helper methods have been moved into the new
class MetricsTest which is used by both the metrics-core and metrics-rest modules.
big refactoring to pull core java metrics out into separate module

The REST server module now has no direct usage of the core metrics API. Nor
does it talk to Cassandra directly with DataStax driver. It now submits
requests to the core server module via the event bus. The REST server module
embeds the core server module.

There are pros/cons to embedding modules vs. running them side by side. That
is something to consider and discuss. The core server module is still pretty
rough. The API for adding metrics is blocking. That will change very soon.