Translation to json

this pull request try to implement #286 .

The goal is to use simple json file to contain translations. That allow to use https://www.transifex.com/projects/p/snap-build-your-own-blocks/ . This web patform is interesting to really disconnect programmers from translators jobs with a nice interface.

In the same time, I changed the place off all languages into `locales/`. I think it makes the repository cleaner.

Before pulling this, I must first integrate all other languages into transiflex and change `translating Snap.txt` with instructions to use transiflex (Actually only French, English and German works)

What do you think about this ?