Possibility to use hierarchical JSON ({"ONE": {"TWO": "Something"}} call...

Hi,

I've made some improvement for my own needs. Feel free to merge them or not. Here are the changes:
* Possibility to use hierarchical JSON ({"ONE": {"TWO": "Something"}} called with "ONE.TWO" key, unlimited levels).
* Wrap missing translations in a span with class "missing-translation" (put a red background on them and you've got something perfect for non-developers who want to help you translate a big application).