language-selection: Allow to select source/target languages

First draft for adding the possibility to select the source & the target language on the results page, while preserving the simplistic home.

I've added some comments/questions in the code. I submit this before finalizing & writing the tests, as I'm not really comfortable with Backbone yet, so it will be good to have a pass of comments first.

Things to note:
* Added the source & target language to the URL, to allow to get consistent results when reloading or sharing a URL
* Based the cache key on the URL instead of the expression, as otherwise it would never only query the server once for any language pair on a given expression