Finish first draft of Dashdash post

Re-added the iPhone images, which I think might be good to use after all.

ARGH at Jekyll's Rdiscount Markdown for not supporting multi-block-item footnotes, so that HTML is a mess and impossible to read, and I had some CSS to make it work.
Replace Dashdash sreencast.gif with YouTube video

Sigh, the hover CSS I'm deleting was a bit of a pain to get working, but it was too easy to accidentally load the image and start the screencast. There's too much going on for the reader not to be able to pause, fast-forward, and rewind.