Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
Now everything makes sense. Vegeta wasn't built to generate that kind of throughput and it's upper bound is significantly lower. Vegeta makes the distinction between load testing (analysing your server under different kinds of load) and benchmarking (testing your server's at peak capacity). It was built to load test individual HTTP services within a Service Oriented Architecture which sustain several orders of magnitude less requests per second.

I am not so sure that Go would be my first choice in building a benchmarking tool in `wrk`'s category but I will definitely play around with your algorithms. Once again, thanks for your input!