Add Markdown syntax support

Add possibility to use Markdown syntax in wiki pages in XWiki for example.

Here as some Java implementations:

• https://github.com/sirthias/pegdown
• https://github.com/cletus/jmd
• http://code.google.com/p/markdownj/
• https://github.com/rjeschke/txtmark/
• http://markdown.tautua.org/