Created data model class for grades.
Added inital grades loading framework.

-Created Grade data model class
-Created GradesLoader to load data into the Fragment
-Created GradesDataSource interface
Removed placeholder TextView from the Grades layout.
Added empty and loading messages to the grades list view.
Hooked up GradesAdapter to the Grades list view.
Added a missing xml file from last commit.
Added Test Data for Grades