Adds a web server to VisTrails, to serve content for web cells

To be able to support web-based visualization frameworks (like d3 or bokeh), VisTrails could serve files from a web server (both the app (html+javascript) and data (csv, json, ...)).

Of course packages need a way to register themselves so they can send data from the HTTP server (simple files, autogenerated/streamed content, xhr/websockets, proxy, ...). As a first step we probably only want to serve static files but we definitely need this to be extensible.