Add the ability to bucket results by latency

I find it very useful to visualize load testing results as a histogram showing how many responses were received within various time buckets. So I added a feature to vegeta to do this.

I don't think this is quite ready for merge. Most of the new stuff isn't tested, and I made some changes in this version to the interface for `NewMetrics()`, which you probably don't want to do.

Basically I'm submitting it now to see if this is a feature you'd consider merging, and to get your feedback on the approach & interface for this feature. If so, we can work to get it to a point where you'd want to merge it :)

At the very least, I'll need to rebase it on the current master.