Parse Markdown and generate HTML at server-side

Use Michel Fortin's PHP Markdown library to parse markdown and
generate HTML at server-side. This would ensure that when a post
loads in MathB, the Markdown content would be already formatted
into HTML properly. This would prevent the user from briefly
seeing Markdown code in the output sheet while PageDown is busy
formatting it at client-side.

Update README.me and tools/setup.sh to automatically clone the
PHP Markdown library in the thirdparty directory.