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).
Sorry, I think my text doesn't reflect all the constraints. Please refer to the javadoc[1], basically all fields used by ```equals``` should be used to calculate the hashCode. Are you using an IDE? They normally have a shortcut to generate ```equals``` and ```hashCode``` automatically using the fields.

[1] https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()