initial changes

added tests

added users_completed

added users_not_started
Merge pull request #270 from edx-solutions/ziafazal/api-course-time-series-metrics-am

course metrics in time series
merged with master
Merge pull request #272 from edx-solutions/ziafazal/api-course-time-series-metrics-am

added modules_completed metric
mattdrayer/gradebook-script-rename: Fixed conflicted command name