Update to latest Rust
Merge pull request #73 from mozilla-servo/rustup_20140618

Update to latest Rust
Upgrade Rust.
Merge pull request #74 from metajack/rustup-20140624

Upgrade Rust. r=mbrubeck
Add a TiledLayer class with Quadtree support
Split out tiles and children
Embed quadtree into ContainerLayer
Have layer and tile lists only store layers and tiles
Allow ContainerLayer to hold extra data for implementations
Allow tracking origin of layers and stop tracking root ContainerLayer in Scene
Make the extra_data member a RefCell so that we can mutate it

We should be able to undo this once the complicated ownership of the
layer tree in servo is sorted out.
Have the Scene hold a ContainerLayer again
Rename tiled_layer to quadtree
Add missing quadtree implementation