Do the right thing when creating statsd counters

You don't need to specify an initial "stats." prefix when creating
statsd counters, since statsd will do that for you. However, this means
that if you *do* add such a prefix, you'll end up erroneously creating a
counter with the name "stats.stats.foo" instead.

This commit strips a leading "stats." prefix if one was specified, but
doesn't error if one wasn't.
Merge pull request #28 from alphagov/dtrt-statsd-counters

Do the right thing when creating statsd counters