Add support for statsd guage delta values

Statsd has supported incrementing guage values since 0.6.0 using +/-
values.

Added support for this by adding a new guage method which accepts a
delta boolean flag similar to the python statsd api.
Updated documentation for previous commit

Updated the readme documentation for the previous statsd commit.
Merge pull request #67 from alirussell/master

Add support for StatsD delta guage values