Changed types of some symbols to fix spacing bugs.

- Added @ as ordinary symbol.
- Made . ? : \ ordinary symbols, not punctuation (see TexBook
  132-4,438).
- Added \colon as punctuation.

Thanks to Tomi Belan.  Closes #24.
Pandoc renderer: Don't put thinspace after Operators.

This often comes before super/subscripts, which looks bad.
See #24.