Improve percentiles computation with HDR histogram

This commit also introduces new fields in the Metrics.Latencies struct.
* Min
* P999
Here are the results for that request rate, suspending the server for 10s.
```
$ echo "GET http://:9888" | ./vegeta attack -duration=30s -rate=200 | tee results.bin | ./vegeta report
Requests      [total]                            6000
Duration      [total, attack, wait]              30.010435746s, 30.009939779s, 495.967us
Latencies     [mean, min, 50, 95, 99, 999, max]  2.170981624s, 139.87us, 786.431us, 9.890955263s, 11.086594047s, 11.354374143s, 11.384070415s
Bytes In      [total, mean]                      66000, 11.00
Bytes Out     [total, mean]                      0, 0.00
Success       [ratio]                            100.00%
Status Codes  [code:count]                       200:6000
Error Set:
```
![screen shot 2014-11-12 at 01 16 26](https://cloud.githubusercontent.com/assets/67471/5003123/9dc6720c-6a09-11e4-8c91-c3ed5655cf3a.png)

You may explore the data further with this [interactive plot](https://www.dropbox.com/s/qy47jezys0aao8g/plot.html?dl=1).