When making easier : use diff to keep old code?

[mr-eggleton]

Currently we comment out all of the old code and append it underneath.
It would be better if we could.

1. In the exercise build make patches between the initials
2. Get a diff of the current code to the initial.
3. Revert back to the initial but then add our change back in on the line above as a comment
4. Apply the correct patch between initials

See http://github.com/kpdecker/jsdiff