Allow for local development, update readme, Jit-Grunt, general clean-up

A bunch of smaller frameworky tasks that needed doing. Including...

* Setting up a local server to allow for development of thrashers without S3. The server gets setup as part of `grunt local`, before anything else happens.
* Update `grunt paths` to return a local snap url.
* Use [Jit-Grunt](https://github.com/shootaroo/jit-grunt) to save space in `Gruntfile.js`.
* Update `README.md` with embedding instructions (cc @stephanfowler).
* Add `/source` to the watch task.
* Add `/source` to `/template`
* Change all sass comments to css comments to reduce size of output.
* Use 4 space indents inside `Gruntfile.js` instead of occasionally two and four.

![200w](https://cloud.githubusercontent.com/assets/1607666/5165242/d82d5148-73d9-11e4-8d75-850ad8530a16.gif)

not seen this before, looks cool