Initial version of new java client for prometheus.

This is based on the existing client, with the following changes:
 - No external dependencies in core client
 - Number of labels is fixed
 - Initilises to 0 when there's no labels
 - Convenience methods for no labels
 - Support for multiple registries
 - Only supports text output format

This code is MVP, there's no validation of metric or label names,
AtomicDouble might be better in Counter/Gauge and I'm sure there's a better
way to architect all this class wise (the api is a bit text format specific
for Summary for example).
The map is being used as a set, so it is better to use a ```ConcurrentSkipListSet```.

[1] http://edelstein.pebbles.cs.cmu.edu/jadeite/main.php?api=java6&state=class&package=java.util.concurrent&class=ConcurrentSkipListSet