Changing doctypes to be an array not object.
Preventing Revision having to be init'd just to validate doctypes

Doctype is now validated in the validators, not in the model.
Changing views to be HBS rather than EJS
Removed JsSketch.Revision & Sketch & Asset; instantiation is now via new Revision etc directly.
Merge branch 'master' of github.com:mattlunn/jssketch-fe
Adding 'Source Viewer' to the FE. This involved adding handlebars to the FE, sharing view helpers between the FE and BE. Also tidied up validators.js and helpers.js in the process (see #5)