Add "Registration Status" section to Class show page

This should close issue #188.

This PR adds a "Registration Status" section to the Class show page, telling Students their registration status and section:

![Screen Shot 2013-02-12 at 7 32 23 PM](https://f.cloud.github.com/assets/1851667/151702/03692b08-758e-11e2-8302-a8ad68c38a99.png)

If the Student has dropped the class, the display is:

![Screen Shot 2013-02-12 at 7 33 12 PM](https://f.cloud.github.com/assets/1851667/151704/2fc29da6-758e-11e2-9ac5-2c7107fff153.png)