anonymous function can not use $this

The MarkDown\Filter\Link class threw an error for using $this in the context of an anonymous function. I added a reference to $this, and made MarkDown\Filter\Link::BuildHtml() public.

This seems to be the best way to stay compatible with php 5.3, as seen on stack overflow here.
http://stackoverflow.com/questions/8391099/using-this-in-php-5-3-anonymous-function

It also mentions the better solution provided in php 5.4