add optional uwsgi support as an alternative to apache

We use the graphite cookbook in situations where running apache is not possible due to conflicts. So we set about to add support for running the web interface via uwsgi under runit.

Right now running as the default will behave as previously (graphite web runs under apache). This doesn't have to be accepted as is obviously. I just want to get the discussion started. I really don't want to keep maintaining a fork.