Implement configurable metric namespace.

This is an implementation of pull request etsy/statsd/#137.
Merge branch 'lambdafu/configurablenamespace' of https://github.com/lambdafu/bucky into lambdafu-configurable-namespace
restore six.PY3 iterators for gauges, counters
use different approach to fixing /dev/shm issue

based on on reading details in travis-core/issues/187
fix syntax nit per Travis test result
more Travis syntax cleanup work
Merge pull request #2 from MrSecure/lambdafu-configurable-namespace

Implement configurable namespaces