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 config (deleteIdleStats) to delete idle sets, gauges, timers - and a overall flag to set them all but allow individual overrides
    update delete_counters unit test to test deleteIdleStats which covers all stat types
update readme and example for deleteIdleStats and friends
reorder example for easier merge
reorder example for easier merge
add a bit more debug output - sets, numStats
remove metric name sanitisation. this is a task for the backends