add 95th and 99th percentiles

@jzoldak @cgoldberg 

Please review. 

This works fine on the test heroku app, but I'm getting a weird error locally that I haven't been able to track down yet.  Can one of you also try it locally to see if it's maybe just my env?

The error I'm seeing is:
```
File '/harstorage/harstorage/lib/Math.py', line 145 in percentile
  f = math.floor(k)
AttributeError: 'module' object has no attribute 'floor'
```