Adding courses

This branch will add the functionality of courses inside the app.

This branch is still incomplete and should not be merged yet.

The biggest change is the creation of a Course::IssuesController, replacing the old IssuesController. Because issues never exist outside of the context of a course, this Controller properly communicates the way that these objects interrelate with each other. 

When you view a course, and thus all of its issues, you are on the CoursesController. But when you click onto a particular issue, you are on the Course::IssuesController.

Things that still don't work:
- A lot of the login system is commented out so I could concentrate on the controller pattern
- Responses can not be added to issues yet through the UI. If you open rails console and manually add a response to an issue, it will be displayed.

Left to do:
Add the ability for responses to be added to issues through the UI, without relying on nested resources
Clean up conflicting code 

Don't merge in yet but please take a peak and let me know your thoughts!
@andersr @samueldowens