Allow using relative links.

This patch adds support for relative links. If you specify a link like

[drupal-relative-link](node/55) or
![drupal-relative-image](system/files/image.png)

this is converted to a link containing a corect drupal url using the drupal url() function:

<a href="/path-to-drupal/node/55">drupal-relative-link</a> or
<img src="/path-to-drupal/system/files/image.png" />

Also urls relative to the document root of the server (starting with "/") are supported. And of course, also absolute urls work.

The relative link problem has already been discussed, and other modules which address the problem exist (like linkfilter and pathfilter). However, those modules all use a different syntax.