Used marked.js instead of showdown.js to render MD

The reason is the rendered result of marked.js IMHO is better than showdown.js when there are errors in the markdown syntax.

The md file that I used for testing is this document:

https://github.com/Codeception/Codeception/blob/master/docs/10-WebServices.md

I believe there are couple of errors on that page, especially on the SOAP section. The html rendered by marked.js resembles closely to the one rendered by github.