requirejs-rails & AMD Compatibility

[requirejs-rails](https://github.com/jwhitley/requirejs-rails) and general AMD Compatibility to handlebars_assets templates

# Adds Config options: 

* use_amd
* handlebars_amd_path - to allow people to have handlebars registered anyway they want with RequireJS

# Enhances `TiltHandlebars#evaluate`

* Now aware of the AMD config options and adjust the compiled template accordingly.
* Now AMD modules are created when `HandlebarsAssets::Config.use_amd = true`.

# Tests

* Sorry - I'll get back to this, but I have a ton of MVP to get done for SxSW... ;)
* WoRkS FoR Me - YMMV