Escape dots in key parts

Hi there,

We're enjoying Pystache at Foursquare and ran into a case where it would be useful to reference key parts containing periods. I came across an accepted pull request in Mustache.js here https://github.com/bitovi/canjs/commit/ffa9afb7b917cc304bb8f52fc850ff26b3c58607 and decided to give it a try for Pystache. Please let us know if you see any significant issues with this change. Looking forward to hearing your thoughts and thanks for your hard work on Pystache!

Best,
Josh