Adding a Grader tab and all associated functionality to the Grade Entry view

Creates a Grader tab in the Grade Entry view, allowing instructors to assign TAs to students on a many-to-many basis. Includes searching, sorting, CSV upload/download, and implements the new permissions in the Grade Entry table.

Fixes issue #49.

![complete](https://f.cloud.github.com/assets/2521245/378321/bb404ba6-a533-11e2-961b-e4dd74f2ceaa.png)