Add clear to Histogram metric

[mbrevoort]

I'm curious why there is not a clear or reset function on the histogram. If you are periodically taking a snapshot to publish to graphite, etc., wouldn't you want to reset the counts and such?

Curious, what are you doing now for transloadit?

[felixge]

You would not want to reset the values if you are periodically reporting to graphite. That being said, I implemented reset() for completeness. I'll rename the clear() methods elsewhere with reset().