support histograms

in accordance with existing codebase, implement this in graphite
backend
clarify bins can be arbitrarily wide
support different histogram settings for different metrics
bugfix: allow bin upper limits to contain decimals

also modify an example to demonstrate
clarify histogram/bin categorisation algorithm
merge branch 'master'
add tests for histograms
put all histogram bins in a "histogram" sub-hierarchy
remove metric name sanitisation. this is a task for the backends
bugfix: make sure histogram metric path is correct

also slight optimisation in the metric setting loop for timers
Merge branch 'histogram' of github.com:Dieterbe/statsd into histogram-integration

Conflicts:
	backends/graphite.js
	exampleConfig.js
make histograms work with the graphite backend
move percentThreshold in global namespace for now
implement graphite test for histograms
remove timer subsection that was never used
clarify histogram graphite test: use a bin that actually includes the value
Merge pull request #162 from Dieterbe/histogram

support histograms