Add default config setup.

Allows specifying configuration in the following files:

* `config/environment.js` - This is loaded in all enviroments.
* `config/environments/production.js` - This is loaded when
  building with `grunt dist`.
* `config/environments/development.js` - This is loaded when
  building a normal development server (via `grunt server` generally).
* `config/environments/test.js` - This is loaded when running tests via
  either browsing to `/tests` URL or running via karma.