Add "How IPython works" overview to development docs

At PyCon Sprints, Fernando gave an awesome overview of how IPython works, but lamented that this isn't documented anywhere. So I thought it would be nice to write up what he said with some figures and add it to the development docs. I had more than a few dangling questions about the diagram and explanation, and I wrote things I was uncertain about in square brackets "`[]`".

As for the diagrams, I considered making them myself in Illustrator/Omnigraffle/theGIMP but thought it was kind of endearing to have someone's handwriting on the internet of all places, so I used a photo from the board to produce all these. The main steps were converting to black and white, upping the contrast to 100%, then clipping out all the remaining grey. If people don't like it, I can always just make it in Illustrator.

The explanation is not fully done yet but submitting the PR now so you can see the progress.