Remove unused receiver names.
Expand the Sample interface and implementations.

Now all the statistics that Histograms report can be driven directly by Samples.
Expose a copy of a Histogram's Sample.

This will allow internally consistent reporting in the near future.
Offload percentiles to the Sample.
go fmt
Make deterministic ExpDecaySample tests possible.
Export generic sample statistic implementations.
Benchmarks show that avoiding allocation is good.
Use Sample to get Histogram stats in reporters.
Add Sum to Histogram and Sample interfaces.
Mostly documentation and naming changes.

Undoubtedly this commit doesn't build or test but it's part of a larger series of intertwined changes.
Fully introduce snapshots for all metric types.
Use snapshots in all the reporters.
Calculate all histogram statistics from samples.

This has the welcome side-effect of now offering a probabalistic recent min/max/etc.
Use the latest interfaces in the Librato reporter.
Merge branch 'samples'