Inline Meter#tick().

Nothing else uses it.
Remove no-op comments.
Added docs for MetricRegistryListener.
Added docs for MovingWindowSample.
Inline Reporter, as it's only used by AbstractPollingReporter.
Added docs and toString impl to Ratio.
Inlined some unused constants in Snapshot.
Added javadocs for GangliaReporter.
Added Graphite#getFailures().

Closes #327.
Formatting fix for GangliaReporter.
Added docs for GraphiteReporter and Graphite.
Added docs for HealthCheckRegistry, plus a few tests.
Added name scoping to InstrumentedHttpClient and friends.
Drop todos from docs.

Too weird.
Move JmxAttributeGauge to metrics-core.
Updated the docs for Metrics Core.