Various improvements based on experience from usage.

- Make descriptions/names in poms useful
- Offer commonly used constants in Collector
- Remove need for casting when using register methods of SimpleCollector.Builder.
- Use Lists/ArrayLists for Collector api, rather than a mix of arrays/Vectors
- Add setChild to allow for easy callbacks for Gauge/Counter
- Add convenience methods to allow setting a Gauge to the current time,
  common in monitoring batch jobs.
- Add convenience methods to make it easier to observe durations