Redis hit ratio

Hi there,

redis hits ration, i.e. hits / (hits+misses) is a really useful measure that we are currently tracking with scoutapp, and it would be nice to also have it in here!

This is pretty much a direct port from the ruby scout plugin (https://github.com/scoutapp/scout-plugins/pull/116) to python. It wasn't tested at all (not really sure how to ...)

And as far as I can tell, adding a new graph on the dashboard happens in NR directly, right?