Add a Gauge class.
Add gauge to the registry and create a Metric.gauge method.
Add gauge to reporters.
Add gauges to the README.
Add a #callback method to the gauge class.
Remove callback method and pass the callback to the constructor.

This looks way nicer and matches how it works in Coda Hale's metrics.
Do not use a default callback and just call it if there is one.
Use a gauge default value of nil.
Merge pull request #33 from bernd/gauges

Add gauge class