New API to get users

Added new API to get enrolled and not enrolled users in Course Content
Group: MCKIN-1027
Changes base on Matt's Feedback

Changes base on Matt's Feedback
Further changes based on Matt's feedback

Further changes based on Matt's feedback
Merge pull request #45 from edx-solutions/ziafazal/users_not_in_group_or_groupmodule_1027

New API to get users