assign application parameters via config files

Took a stab at using config files to assign application parameters. Mainly just trying move the conversation on this. Some thoughts:
* it would be more elegant to extend a parameters object than assigning values by reference, but that would create a more substantial style difference.
    ```javascript
    var App = Ember.Application.extend(window.ENV.CONFIG, {});
    ```
* modulePrefix only needs to be defined once. Maybe that doesn't belong in a config file.

Split from #468