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
    fix the deleteCounters bug where packets_received and bad_lines_seen are deleted - cannot ++ an undefined value..

    adjust how we check for the stats counters to support the prefixStats patch - should apply safe with and without the patch

    correct delete_counters unit test to not look for bad_lines_seen of NaN instead look for it set to 1
add a bit more debug output - sets, numStats
remove metric name sanitisation. this is a task for the backends
remove commented lines
Merge branch 'etsymaster' into p/debugupdates
update debug log to new logger, make all debug prints use consistent DEBUG: prefix by removing the extra arg
fix typo 'stats_count.' -> 'stats_counts' and reword for clarity