First Round of Feedback from Phil

Major Changes:
- Simplified terminology across codebase to make consistent.
- Expanded docstrings to talk about outcomes.
- Removal of Joda Time and SLF4J.
- Added blackbox workflow tests.
- Registration framework for adding exposition hooks.
- Inclusion of HotSpot VM metrics.
- Support non-annotation registration.

Pending:
- Metric Builders and Partials are immutable and can be used as
  templates.
- Full exposition of HotSpot VM metrics.

Infeasible:
- Removal of Guava.  GSON depends on it transitively, so I want to
  keep my dependency explicit.  Guava is unlikely to have many
  interface changes between releases.  Plus, it provides AtomicDouble.
Enable immutability in Metric.Builder.