Various enhancements for pre areas
Various enhancements for pre areas

- Limit pre area's height and allow using a vertical scrollbar for overflowed
  content (I think this might be considered as the most appealing reason for
  using html slides involving codes).
- Render a nice vertical scrollbar for overflowed pre areas.
- Add a few space after linenos.
- Support Github's Gist for consistency.
- Add new fixed width fonts to prevent falling back to Courier.

adamzap/landslide,kwangkim/landslide,exciteresearch/landslide,ptisserand/landslide,ionelmc/python-darkslide,kwangkim/landslide,adamzap/landslide,kwangkim/landslide,exciteresearch/landslide,ionelmc/python-darkslide,ionelmc/python-darkslide,ptisserand/landslide,ptisserand/landslide,adamzap/landslide,exciteresearch/landslide