Created CourseFragment as a base class for Fragments whose data depends
on the currently selected course.  MainActvity now implements
CourseDataProvider.
Created data models for Assignments, Documents, and Blog Posts.  Created
a data model for Stream Items.
Added CascadeDataSource which manages retrieving data from the Cascade
server.  Created Course data model object for holding course data.
Modified Course and Assignment to use the Builder pattern for
constructing immutable objects.  Started CascadeDataSource interface.
Added argument checking to Item constructor.  Added documentation to
Document data model class.
Added BlogPost methods to CascadeDataSource interface.  Modified item
id's type from long to int.
Created Added static modifier to Item subclass Builders.
Added FakeDataSource which implements CascadeDataSource to provide test
data.
Added a CascadeDataSource field to MainActivity
Merge remote-tracking branch 'git-hub/master' into fragment_communication