Statsd flexible histogram

The collectd Statsd implementation currently will only accept values in the range from 0 - 1000 in the histogram it uses to calculate the percentile. In case of of values > 1000 these will be ignored; the Histogram is to be compared to an over exposure photo. 
To be able to adopt to other usecases the range of the histogramm has to become flexible so the user can configure the range of values his data series generate.

(fixes #401)

I think "bucket_size" would be a better name than "scale".