stats_accum_input cleanup

Hi I've cleaned up the implementation a little bit:
 - The calculation of timer percentiles was completely wrong so I fixed that. 
 - I've prepared the implementation to easily support setting custom namespaces/prefixes via config (I plan to add it later)
 - I've added tests of computed timer fields

Once this PR is accepted I plan to make this implementation on par with: 
https://github.com/etsy/statsd/blob/master/backends/graphite.js

just FYI this is my 2nd time using Golang :) and my first PR ever :) so please be gentle ;)