Statically serve php files with 'jekyll serve'

In order to integrate a "upcoming events" list to the frontpage of our website, we employ some webdav php code in index.php. However, "jekyll serve" does not ship php files for two reasons:
- The mime type for php is not set (to text/html), so index.php would be downloaded by the browser rather than displayed.
- index.php is not in the DocumentIndex when launching webrick.

This pull request fixes this. Note that by these little changes webrick still does not run a php interpreter, however, for most authoring tasks (adding static content, adapting layout, etc) it is fine if the php code is displayed rather than not serving php files at all. So this is a major improvement for our workflow.