Add grading to the mentoring XBlock
add a weigth property to mentoring Blocks
make the max grade (weight) a xml mentoring attribute
use a simple Field attribute for weight now that type is enforced
Merge pull request #11 from FiloSottile/grading

Add grading capabilities to mentoring blocks
max_attempts should be global to the mentoring block
various minor fix
No more need to cast with the enforce_type in XBlock