add validation for grading types (allow only integers)
STUD-988
Merge pull request #2052 from zubair-arbi/zub/bugfix/std988-integergradingtypes

add validation for grading types (allow only integers)
The function tzAbbr() causes null-reference exception when the browse returns a non-English timezone string.
Add 'Yihua Lou <supermouselyh@hotmail.com>' to the AUTHORS file.
Merge pull request #2060 from edx/louyihua/tzAbbr

Fix tzAbbr() in non-English browsers #os