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
Add two addtional graphite backend metrics
Line up concats
remove last_ prefix to avoid admin 'stats' match
add helper script to decide which timers to sample down
description fix
Fixing global leaks
Another global leak
remove orphaned variable
examples: python: added python3 support
examples: python: fixed batch update_stats()

This fixes nasty bug introduced by:
    examples: python: use isinstance() to suport inheritance

Pointy hat to: me
TODO: proper tests for update_stats
examples: python: use .format
examples: python: split sample and send

This will simplify testing