Facet ranges

Hi there, 

So I wanted to have a facet ranges, so, filtering products between £10 and £30, for example.

In development I've got this working as is:
https://www.dropbox.com/s/i2lea5cq1kh2zjx/Screenshot%202013-11-22%2015.00.50.png

I've added passing tests in there too

Andy.