Do not create an empty node for obj files with only one model.
Add the rust logo to the `obj.rs` demo.
Add a demo `decomp.rs` that loads an obj file and displays its convex decomposition.
Update to the last rust-nigthly.

Version of rustc: 0.12.0-pre-nightly (01ec6fab2 2014-08-18 00:46:10 +0000).