Add statsd client and time _get_user

There are some concerns over the amount of time that stagecraft is
taking when talking to signon. This commit will time the function and
log it out to graphite.
Merge pull request #181 from alphagov/add-statsd

Add statsd client and time _get_user
This should be using django defaults not env

I accidently pulled in the defaults from environment variables rather
than the django settings but had set the vars in django settings. This
brings statsd into line and means we should now get the correct prefix.
Merge pull request #182 from alphagov/add-statsd

This should be using django defaults not env