Have all intermediate Python notebooks use same data files

Do to changes made after the introductory notebook was created, a
change in filename structure and contents was needed to keep the
lessons simple. This updates the intro lecture to use the same
files as the following lessons.
Rerun intro intermediate Python notebook using new files
Remove old intermediate Python data files
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
Modifying Mac git install directions

The first line says to download git, but this only works for OSX10.8
and higher. This fix makes clear that this is only for OSC 10.8 and
encourages students to read the second line to find versions for 10.5
- 10.7
Changing wording at Raniere's suggestion
The language "use the most recent available" was confusing to me.
I added "for your OS"
Move SSH Key lesson from Git to Extras

Close #691.

After some discussion about have and where to put the lesson that
cover SSH Key I and @wking agree to move it to
`novice/extras/06-sshkeys.md` and just add a note + links to help
students set up their own SSH Key for the Git host they will be using.

This commit includes @jdblischak and @wking wording about HTTPS vs. SSH.
Fix typo in Novice Python 04
Merge pull request #715 from ethanwhite/typo

Fix typo in Novice Python 04
reviewed
added mulit-choice
remove confusing anglification of time machine
changes as suggested by @ctjacobs
fix ../../gloss.html link
in and out burgers, err blocks.