Gauges are constant until overwritten, and not reset at flush time.
Implement configurable metric namespace.

This is an implementation of pull request etsy/statsd/#137.
Merge branch 'lambdafu/configurablenamespace' into lambdafu/master

Conflicts:
	bucky/statsd.py
Merge branch 'lambdafu/gaugefix' into lambdafu/master

Conflicts:
	bucky/statsd.py
Implement gauge delta updates.