Markdown links can be resolved to content tree nodes

I ran into a limitation of the way the Markdown plugin resolves url :

 * it works well when linking to a file in the same directory as the .md file (showing an image for example)
 * it does not work when linking to another node of the content tree (a link to another article for example)

This PR changes the way the links are resolved by the Markdown plugin in order to accept the following :

     * [absolute url to wintersmith website](http://wintersmith.io/)
     * [relative to the current content tree node](../bamboo-cutter/index.md)
     * [relative to the content tree root node](/articles/red-herring/index.md)
     * [anchor on the content tree](../markdown-syntax/index.md#link)

All other cases (error cases for example when the path is not in the content tree, basic anchors) keep using the previous algorithm

I modified the blog "hello world" article to list those cases.

I hope you will be able to review it and hopefully accept it !