Make the book variable width

[jbeckers]

On my 1920x1080 screen, the book is rendered 2546.2 px wide, with a horizontal scroll bar. Resizing firefox makes no change, the book is fixed width.

When printed, the text is illegibly small.

Without display: table rendering is greatly improved:

[jbeckers]

Note that the large width might also be fixed by #105. I'm by no means a css expert. In fact: I have no idea what I'm doing. 😄