Bootstrap3

So,
This is a move to Bootstrap 3, not perfect because of two problems:

1. I tried to setup the bootstrap-sass gem, but it needs some backported gems to work with rails 3.2, and generates assets path issues in the rails engine. It would be best to choose it after upgrading to rails 4. Meanwhile I used bootstrap-sass-rails, which is deprecated (well rails 3.2 is, isn't it) since feb.14.
2. bootstrap and formtastic integration here : https://github.com/mjbellantoni/formtastic-bootstrap . For a reason, the formBuilder raises a RuntimeError when trying to retreive default hint or error class . I will contact the repo owner to get an explanation about this strange behaviour.  I included a monkey-patch in the engine. We may participate to the effort by contributing to this gem, Formtastic BS3 integration is seriously wanted.

We can now work on the views, but this PR needs some rework after upgrading to rails 4.