Deliver original demo.css as pagedown.css

Easier to get things going when having this bit packaged together.

The pagedown.css is copied from
https://code.google.com/p/pagedown/source/browse/demo/browser/demo.css
with the following changes:

* .wmd-panel is commented out. It garbles my layout, YMMV.
* .wmd-button's background-image is set to "/static/wmd-buttons.png"
* whitespace cleanup: trailing whitespace, tabs->spaces, curlies