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).
I've had unittesting in mind when designing this.

This client allows for you to specify a registry when registering metrics, so should be much easier for unittesting - that's what the client unittests themselves do for example. There's also the getSampleValue function for integration testing, and you can get() the metrics to read their values for unittesting.

I want to keep the default registry to make usage easy, you're not required to use it.