Handlebars v2.0

Hi,

thanks for creating this module, I've been using it alot! This updates to handlebars v2.0. As its still in alpha you should probably not merge it just yet, hopefully a stable release will happen soon.

I'm using this version myself as I have the need to retrieve data from the root context in my templates. Support for retrieving data from root was added in v2.0.0-alpha.1 with this syntax `{{@root.foo}}`. Read more about it in issue wycats/handlebars.js#392.

Tip for anyone else who wants to test handlebars v2.0 in their projects:
```bash
$ npm install git://github.com/phillipj/express-hbs#feature/handlebars-2.0
```