Organize the modules directory

[brockboland]

Contrib modules should go in a contrib subdirectory, so that we can also add custom and features directories alongside it, and keep track of where modules come from.

I need to confirm that Drupal won't go down in flames if it doesn't find those files where it expects to find them. I think we just need to clear the cache and rebuild the class registry, but that needs to be tested.

[brockboland]

Turns out, this is going to be more complicated than I thought, so screw it.