Filter for contractors

[blobaugh]

We need to implement a filter for the contractors list. This should be a simple form that lets site visitors choose to see contractors based on the "I can do" portion of their profile.

It should sit at the top of the contractor list in the shortcode.

Would be great if the page magically updated via ajax when the user alters the filter, however we still need to ensure the query winds up in the URL bar for easy sharing. If we can meet those two conditions it would be fantastic!