Update client to reflect new API needs.

1. The output format is now versioned per the Semantic Versioning scheme.  Mainline Prometheus will be adapted to use differing consumption methodologies as the generators' formats evolve to support legacy clients.

2. The telemetry outputter now supports GZIP output encoding.  In sample runs, this cuts the output size in half.

3. Basic sanity tests are added for registration with varying levels of pedanticness.

4. We have support for base labels in the registration and emission phases.

5. We have label support for individual metric mutation operations.

6. A number of simplications have been made.
Merge pull request #13 from matttproud/feature/labels/base-exposed

Update client to reflect new API needs.