Add client methods for new data API endpoints

This implements client methods for the sequential open distribution, grade distribution, and answer distribution API methods which were merged into the data API a few weeks ago.  It also includes test cases for the new client methods.  @dsjen and I are planning to use these elsewhere in the platform in the near future, so it would be nice to have some reviews on it this week when you guys are free.  As always, feedback is encouraged.  Thanks everyone! @jbau @clintonb @rocha