Update pagedown library: rev. f4ebd25329ec

Fetch from rev.  f4ebd25329ec

Markdown.Converter.js - f4ebd25329ec Apr 24, 2014
Markdown.Editor.js    - 9087581ade21 Jan 26, 2013

https://code.google.com/p/pagedown/source/browse/?r=f4ebd25329eceea528734639b21f74a4b1906996
Import markdown extra plugins for pagedown: rev. bd0870e3e8

https://github.com/jmcmanus/pagedown-extra
Patch pagedown extra: add preclass for highlight.js
Load pagedown-extra with default options
Add sample page to explain pagedown extra syntax
Update document to mention pagedown-extra plugin
Fix failed test due to adding pagedown menu
Add pagedown extra plugin into MANIFEST
Remove mis-committed closing div tag
Merge branch 'feature/add-pagedown-extra' of github.com:keedi/Galileo into keedi-feature/add-pagedown-extra
move embedded JSON structure to its own variable