Prevent output of headers by the property_details shortcode

The [property_details] shortcode adds an “Additional Features” heading at the end of the property details even if no features are attached to the listing in the Features meta box.

It also adds outputs headings in the property details section even if details aren't added (issue #6 ). This PR includes a fix for both.

I recommend also updating AgentPress Pro to include this CSS to hide the first column if there are no entries in it and avoid display issues like this: http://d.pr/i/1jk2E

    .property-details-col1:empty,
    .property-details-col2:empty {
        display: none;
    }

`:empty` doesn't work in IE8 or below, but it should improves things for modern browsers.