Update to the last rust-nightly.

Version of rustc: 0.12.0-pre-nightly (82c052794 2014-09-05 17:36:25 +0000).
Add margins to the Rigid Bodies.

ncollide recently removed the concept of margin from its geometries.
Thus we have to emulate them here to get better performances (but less accuracy).
Update to the last ncollide api.
Update to the last rust-nightly.

Version of rustc: 0.12.0-nightly (d64b4103d 2014-09-26 21:47:47 +0000).
Fix .travis.yml.
Update to the last nalgebra api.
Update to the last ncollide build system.

As a side effect, this splits the `examples` directory into two directories `examples3d` and
`examples2d`.
Update to the last rust-sfml api.

Version of rustc: 0.12.0-nightly (b5ba2f551 2014-10-06 20:27:14 +0000).
Fix warnings.