Add test to expose data race in Histogram.Count
Fix data race in Histogram.Count
Reduce slice re-allocations on unfilled Sample.Update
Fix reservoir sampling algorithm for uniform sample

Old variant was representing a moving window.

See also: https://github.com/rcrowley/go-metrics/issues/56

http://en.wikipedia.org/wiki/Reservoir_sampling
Use RWMutex for samples
Uniform sample doc. attached to public function