Converting to individual challenge div's in the novice shell lessons
Converting to single-div-per-challenge in novice Git tutorial
Bugfix: missing file `fractal_1.txt` replaced with `small-01.csv`
Fixed problem with range_overlap tests in ipynb file
Fixed problem with range_overlap tests in .md file
Updated an error message
Removed reference to the reverse method in ipynb file
Removed reference to the reverse method in md file
Add GitHub Ribbon to our lessons

Backport of 9a19e91 from swcarpentry/site.

All credits to Abigail Cabunoc <abigail.cabunoc@oicr.on.ca>
for the awesome work with CSS.
Merge pull request #525 from zonca/formatting_fixes

Bugfix: missing file fractal_1.txt replaced with small-01.csv in python lesson 6
Edited wording of reverse challenge in ipynb file
Edited wording of reverse challenge in md file
Put 3 spaces after bullet points
Merge pull request #526 from DamienIrving/lesson_fixes

Some minor fixes to challenges in some lessons
updated discussion list URL

Current URL for the discussion list takes the user to a 404 page

current: http://software-carpentry.org/contrib/discuss.html
proposed: http://software-carpentry.org/pages/discuss.html
Merge pull request #528 from benmarwick/fix-typo-in-contrib

updated discussion list URL
Merge pull request #514 from gvwilson/every-challenge-in-its-own-div

Put each challenge exercise in a div of its own.
In the loop challenge changed the example: 2**4 becomes 5**3.

During a bootcamp, I was staring at 2**4 and realized it is ambiguous (in the sense that 2**4 == 4**2) and thus can be confusing for students.
It is not a big deal but it is easy to fix so here it is.

NB: I'm not committing the generated md file as it seems to contain more changes (added cstyle attributes) probably due to my ipython version.
Improve CSS for GitHub ribbon

- For small screen, preserve margin.
- Use css-beautify
Merge pull request #531 from twitwi/fix-ambiguous-python-exponentiation

In the loop challenge changed the example: 2**4 becomes 5**3.