Add support for JS namespacing

Hi, 

I'm working on embeddable scripts. Here is an example :
```html
<script language='javascript'>
  (function(){var a=document.createElement("script");a.type="text/javascript";a.async=true;a.src="//js.sharypic.com/widget-loader-1.0.js";var b=document.getElementsByTagName("script")[0];b.parentNode.insertBefore(a,b)})()
</script>
```

By using I18n as it was, I Could create conflicts with the webpage hosting my embeddable code. Let's imagine a window.I18n already exists, It would just override things and broke stuffs.

So I needed a way to scope it. Here is a proposal: within I18n-js.yml add:
```yamp
js_namespace: myawesomenamespace
```

Then, the vender/assets/javascript/I18n.js plugin will be scoped as  (same thing with the translations.js.erb file):
```javascript
var myawesomenamespace = myawesomenamespace || {};
myawesomenamespace.I18n. = myawesomenamespace.I18n || {}
```


Saying that, It's not mergeable in this state. I fought some time with the rake spec:js in order to precompile the I18n.js.erb file, but can't figure how to make it works before runnings the spec. So if you are interested by this pull request, I would love to hear  about the spec-js gem to bullet proof your JS tests.

By & great works for this gem