Regex notebook preview

There's a lot of material I want to add to this before a real merge but I wanted to give everyone a chance to preview the regex material I've written so far. [Rendered Notebook](http://nbviewer.ipython.org/urls/raw.github.com/jiffyclub/notebooks/regex/regex-intro.ipynb)!

I'm using the existing regex video as inspiration but approaching things from a different angle. In particular I'm introducing things with the `re.findall` function instead of `re.search` as in the video. This makes it a little easier to see the results of regex actions without explaining a `MatchObject` object first. I've also deferred the introduction of groups a bit, though that'll be the next topic in the notebook.

Let me know if you've got any feedback and I'll keep going on this as I have time.