added Markdown support libraries for PHP versions under 5.3 (tested on 5.2.17) and PHP version check.

re #63 (sebsauvage/Shaarli)

PHP Markdown libraries downloaded from http://michelf.ca/projects/php-markdown/classic/
Current versions are:
  * "PHP Markdown Extra" (currently v1.2.7)
  * "PHP Markdown" (currently v1.0.1q)
re #63, fix CSS styles to support Markdown in link descriptions