Collaborate in pairs for the git lesson

The novice git lesson currently teaches collaboration by getting
students to clone the repository to a second directory on their own
computer. For the August 2014 workshop in Cambridge, I made these
changes, to put students into pairs instead and have them collaborate
between two computers.

This is somewhat subjective, but I think it's an improvement.
Collaborating between different people and different computers is more
realistic, so students should be better motivated. They learn an extra
step, to add collaborators in the Github UI. Working in pairs probably
also gets students talking and helping one another more.
Add a note about how to work through the lesson on your own
Fix a couple of bits I missed before.
Restore $ prompt that was accidentally removed
Delete unused git status diagrams
Merge pull request #694 from takluyver/novice-git-collab-pairs

Collaborate in pairs for the git lesson