Various style fixes:
- Main content floats below sidebar if the window's too narrow.
- Table of contents needs styling with enclosing box.
- First and second level headings need unique styles.
- A ";" at the start of a line doesn't make the line bold.
- Floating images need more padding.
- Scrollbars appear in weird places, such on each line of a diff.
Merge pull request #2 from blob8108/css-fixes

Various CSS fixes