Add bounding sphere computation.
Start the addition of bezier surfaces and curves.

Collision detection is not yet implemented for anything but `BezierSurface` vs. `Ball`.
Ray-casting is not implemented (it always returns `None` for now).

This also adds a new utility (needed for bezier curve/surface subdivision): `VecSlice/VecSliceMut`.
It is a wapper on `Vec` that give access on regularly-spaced elements using natural, continuous
indexing.
Add a cached subdivision tree to speed up the ball-vs-bezier collision detection.

However, it is not activated yet.
Also, this updates ncollide to the rust-nightly:
0.11.0-pre-nightly (faa7ba7 2014-05-31 01:06:40 -0700)