Mentoring assessment mode

The work of assessment mode. 

Currently missing:

* Taking into account the max_attempts
* Make all light children have their own *weight* for score
* Test in client app 

CC @antoviaque if you want to review and give it a try in the LMS
Typo

Also CC'ing @mtyaka on the changes to the README, as he's updating the rest of the doc for the mentoring XBlock