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
Init. commit of (medical) image processing ipynb.
Add height to Eventbrite iframe

and values for when using two rooms.

**The height provided isn't suitable for small screens.**

Close #493.
Made changes suggested by Will Trimble - change Lena for Sally Ride.
Downsampled image to sensible size and removed references to SciPy Lena.
Simplified introduction by removing complicated colourbar placement stages.
Finished cleaning up IPYNB to show Gaussian fit to background.
First pass at normalizing novice shell files
Finished getting all the right files together
Finished making text match filesystem
dotfiles still show up in the find exercises
Updated diagrams to match text
Miscounted - it's still seven directories.
Some fixes suggested by @claresloggett
Remove description of strides from novice Python 01

The current section on slicing is already relatively long. Since
we don't use strides anywhere else in the novice Python material
this removes them to avoid overwhelming the students with too much
detail too early.
Updating markdown version of 01-numpy
Remove reference to strides in natural numbers exercise

The ability to skip over count in numbers other than one isn't necessary
for the exercise, so this presents the minimal amount of information
that the student needs.

This also allows us to remove discussion of strides from the discussion
of indexing in the 01 notebook letting us minimize the amount of extra
material we are delivering.
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.