Fixing #25 (Multi line code listings break on auto-reload)

The fix works by replacing the local showdown.js with a dependency on the 'marked'
markdown renderer (https://github.com/chjj/marked) which seems more close to what
github.com is doing.

While the code blocks are not syntax highlighted as github does it still produces a ```<pre><code>...</code></pre>``` output for multi-line listings, fixing at least the multi-line issue.