Turn on parallel builds.

The bugs are believed to be worked out. From a `make clean`, `make` took 8:41
and `make -j6` took 4:43.
auto merge of #902 : metajack/servo/parallel-all-the-builds, r=jdm

The bugs are believed to be worked out. From a `make clean`, `make` took 8:41
and `make -j6` took 4:43.
Make sandboxed iframes run with different script tasks.
Add trace hooks for Window and Document, and remove explicit rooting for the root DOM node. Fixes #901.
auto merge of #900 : jdm/servo/sandbox, r=metajack

The rust-mozjs change is just updating it to tip; do not be alarmed.
auto merge of #908 : jdm/servo/gccrash, r=metajack
Implement getters and setters for img width and height properties
Longcat demo
auto merge of #912 : brson/servo/longcat, r=jdm

This also comes with a longcat demo, including about 35k of images that I don't know the copyright status of.