DOC added testimonials

from INRIA and evernote

TODO list

- [x] Check with evernote they are OK
- [x] Get 4 testimonials (current 3 / 4)
- [x] Activate the testimonials on the main page
- [ ] Set the carrousel to display two logos.

Ping @GaelVaroquaux and @ogrisel 
refs #1370