ModulestoreEnum class.
Adds is_xblock_visible_to_students
Fix branch settings.
Fixed IE security warning on course page.
LMS-2892
Merge pull request #4232 from edx/nimisha/modulestore-enum-constants

ModulestoreEnum class.
ORA2 release-2014-06-30T13.39; update EASE to bump the NLTK version to 2.0.4
Merge pull request #4215 from edx/waheed/lms2892-ie-security-warning-fixed

Fixed IE security warning on course page.
Merge pull request #4252 from edx/will/ora2-release-2014-06-30T13.39

ORA2 release-2014-06-30T13.39
Merge pull request #4229 from edx/studio/uses-mixed-modulestore-xblock-visibility

Adds is_xblock_visible_to_students
Uses ModuleStoreEnum in is_xblock_visible_to_students
Do not refund student if a certificate entry already exists for them.

LMS-2920