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.

Some of the diagrams that the lesson was previously using are no longer applicable. I added a couple of new ones in their place.
You could keep

~~~
$ cd /tmp
$ git clone https://github.com/vlad/planets.git
~~~

since `/tmp` is a valid directory .