Async translations with multiple fallback languages
wip(): teaching $translate asynchronicity
Merge branch 'fallback-languages' of https://github.com/Sanjo/angular-translate into sanjo

* 'fallback-languages' of https://github.com/Sanjo/angular-translate:
  Async translations with multiple fallback languages
Documented the new code for async translations with multiple fallback languages
Failing test for: it should load a fallback language asynchrously only once
Merge branch 'fallback-languages' of https://github.com/Sanjo/angular-translate into sanjo

* 'fallback-languages' of https://github.com/Sanjo/angular-translate:
  Failing test for: it should load a fallback language asynchrously only once
  Documented the new code for async translations with multiple fallback languages