Move all external dependencies to submodules.

We should have in our repository only code which is of our doing.
This commit moves removes all code which is not ours and loads it
from the external repositories as submodules.

The only exception is scripts/async.js. Is it the one from
https://github.com/fjakobs/async.js ? It seems to be different.

Signed-off-by: Matěj Cepl <mcepl@redhat.com>