Implement remote resource loading

mv.loader.use takes an array of URLs, which are fetched via d3.xhr.

This also restructures mv.loader to refactor out some shared functionality
between mv.loader.use and the file selector change events.
mv.loader could still use some cleanup.

http://css-tricks.com/examples/ButtonMaker/ makes a pretty neat css button.
Hopefully, it's visually attractive enough so people in a hurry know where
to go. :)