implementation/tests of the template inheritance mechanism 

hi @fw42 & @tylerball,
Based on the conversation here (https://github.com/Shopify/liquid/pull/264), this is my implementation of template inheritance, similar to what offers Django (https://docs.djangoproject.com/en/dev/topics/templates/). 
Code mainly extracted from the LocomotiveCMS fork of Liquid. 
Feel free to comment and ask me questions.
Thanks!