Add global helper

[Fauntleroy]

We could probably use a helper to provide access to variables that are out of the current scope. Since we usually need to just access something at the top of the current context, a simple global helper would do the trick.

<img src="{{global "url.host"}}/images/banners/approach.jpg" />

Inspired by this SO issue: http://stackoverflow.com/questions/18856023/handlebars-js-global-contexts