Made it possible to use fallbacks for modules

Now configuration like this

```yaml
paths:
  jquery:
  - http://cdn1.com/jquery-1.9.1.min
  - http://cdn2.com/jquery-1.9.1.min
  - jquery-1.9.1
```

works as expected: adds `empty:` into build_config for such module
and adds this paths directly into config of require.js on page.
Make sure jquery-1.9.1 is precompiled on your server, when using local fallback.