Adding include_gon_amd, an AMD compatible version of include_gon

Adds `include_gon_amd`, which is an AMD compatible version of the default  `include_gon`.

I'm using gon in a project that uses RequireJS so I needed a way to define the variables in an AMD module. So here goes.

I first thought of implementing the functionality as an option to the default `include_gon` method, but opted for a separate method since not every option that `include_gon` supports (`namespace_check`) makes sense in the AMD context.

Very simplified usage example, using requirejs-rails

``` ruby
# in app layout file
requirejs_include_tag 'application'
include_gon_amd namespace: 'data'
```

```coffeescript
# a (coffeescript) module that requires some data
define (require) ->
  data = require 'data'

  console.log data.comments
```

I'm ready to fine-tune the api and add more specs, if required. Is this something you'd be willing to merge?