Use Parsedown instead of Texy for markup.

This branch replaces Texy with Markdown for markup parsing. Markdown is the de-facto standard for documentation (see Doxygen, phpDocumentor) and widely used. Using Markdown would instantly make changing to ApiGen from other documentation tools very easy.

[Parsedown](http://parsedown.org/) is used as the Markdown implementation of choice. It is production ready, very fast, precise and well tested. The changes are minor and implementation was simple.

There are the following issues:

-   There does not seem to be a PEAR package for Parsedown.
-   Code blocks starting with four spaces do not work. I suspect that leading whitespaces are stripped. Fenced code blocks using ``` (three backticks) work.
-   The allowed HTML tags are neither checked nor enforced.
-   The documentation is unchanged.

These issues are probably very easy to work out for someone who understands how ApiGen works internally. Even if you do not accept this request, please let me know if you have an idea on the code blocks issue.