Enhancement: inject logger object into backend

 * allows backends to use/consume the configured logger from statsd
 * this is optional, backends who don't use it yet, should continue to work
 * related: librato/statsd-librato-backend#17
Merge branch 'master' into topics/inject-logger-into-backends

Like a boss.
Enhancement: update backend documentation

Document the new logger parameter.

cc @mheffner
Enhancement: consume logger in graphite and repeater backends
Merge pull request #369 from easybiblabs/topics/inject-logger-into-backends

Enhancement: inject logger object into backend