Refactor out base url into config, in case we re-deploy our backend elsewhere

This:

``` coffeescript
class SomeModel extends Backbone.Model
  urlRoot: "#{ config.BASE_URL }/bills"

```

Not this:

``` coffeescript
class SomeModel extends Backbone.Model
   urlRoot: "http://omnibus-backend.azurewebsites.net/api/bills
```
