Replace images and webfonts tasks with grunt-contrib-copy

First stab at using grunt-contrib-copy for copying over images and fonts.

It's nice to get rid of those custom tasks. A couple downsides though:

1. In order to keep the `images` and `webfonts` task names, I had to use `renameTask`. (aliasing the tasks to `copy` doesn't work the way you'd expect) And in order to rename the copy task twice, it has to be loaded twice. Not a huge deal, just a little quirk in the load-stuff module.
2. Due to the way the copy task handles the `cwd` file option, we'll have some funky config values in the `files` object. (Though this pain would actually be eased a bit if we were to extract a generic config option for setting the `app` and `vendor` directories.)