Implement @root property

Handlebars added an @root property to look up the root context in
https://github.com/wycats/handlebars.js/commit/cb80f4684323f2924d4251eeb3d7b12cc37ea6d
This adds equivilent support in lightncandy in a non-invasive manner.
Merge pull request #26 from ebernhardson/access-root-context

Implement @root property