basic rabl template engine

This engine renders the given path, in the source path, using Rabl.
Rabl is in "hash" mode so that only raw data is returned.  Sanford
then takes the raw data and serializes it to BSON as needed.

This should be all that is needed to have a working sanford rabl
template engine.

Related to redding/sanford#85.

@jcredding ready for review.  Obviously you can see I decided to go ahead and implement a rabl template engine for Sanford.  In the end I figured it would make the transition off of Rabl easier and it's more example code out there for working with Sanford template engines.