Tweak pandoc.css

Eliminate horizontal lines except below `<h1>` and `<h2>` tags. Still
looks a bit yucky.
Update lecture 2

Include static images, eliminate unnecessary calls to `render`.
Update lecture 5

Eliminate unnecessary calls to `render`
Update lecture 6

Eliminate unnecessary calls to `render`, reformat the menagerie of
surfaces, lowered default resolution (after noticing that iPad 4 with
iOS 8 Safari chokes and crashes at given resolution).