Add knowledge

This will add a knowledge attribute to players, and adds courses at the university that the player can enroll in.  One of the courses (option 4) requires a certain level of knowledge already and has a prerequisite class before it will show up.

To test:

1. N for new game
2. A to add player
3. Name your player
4. S to start game
5. M to move
6. U to move to the university
7. C to enroll in a course
8. 1 to take Basic math
9. C to enroll in a course
10. 2 to take Basic reading
11. C to enroll in a course
12. Verify that option 4 is now available

Optionally, if you run the game in debug, it should print out the knowledge of the player as you go.  You can verify that the knowledge goes up as courses are taken.

This fixes #35.