Add backbone.js as a submodule.

By this we can now update versions of backbone.js more easily.
Currently, backbone.js submodule is checked out at 1.0.0 tag.

What do you think about this? I mean this pull request more as a suggestion for discussion rather than final complete solution of the problem of updating.

Signed-off-by: Matěj Cepl <mcepl@redhat.com>