Made it possible to use fallbacks for modules

Now configuration like this

config/requirejs.yml:
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: replace paths array with "empty:" in build_config for such module
and adds this paths directly into run_config on page.
Make sure jquery-1.9.1 is precompiled on your server, when using local fallback.