Expose Timer Histogram percentiles from DropWizard as metrics

[tallpsmith]

I'm going to be adding a couple of extra metrics to the Parfait-DropWizard extension for Timers to expose the histogram percentile's for request latency measurements. Currently it only exposes the request rate counter, and 1/5/15 minute rates.

On the request latency it provides an average, median, min/max,stddev, but that's over the lifetime of the process which isn't that useful for very long running webapps with often long durations of 'less busy'.

The existing DropWizard metrics provides histogram decaying reservoir percentiles for 'recent' requests in the classic, 75th/95th/98th/99/99.9th Percentile which exposes much more useful 'recent' distribution information about requests.

These will appear as additional sub-metrics alongside the existing.

I'll be doing this in a new v3 branch, and merge back to master (Babylon is not yet migrated to the new v4 namespaces as yet).

[tallpsmith]

Completed in 4.x (master) and 3.x branches (though with different namespaces..)