Add support for gauge deltas.

* Add tests for gauges and new gauge deltas.
* Add support for +/- prefix for gauge values to support modifying
  gauge values in-place.
Remove numStats asserts.
Merge pull request #72 from jsocol/modify-gauge

Add support for gauge deltas.
Update readme with gauge delta info.
Merge pull request #256 from jsocol/gauge-readme

Update readme with gauge delta info.