auto merge of #464 : Sicki/servo/patch-2, r=metajack

without: 
libGL error: failed to load driver: vmwgfx
libGL error: failed to load driver: swrast

platform: ubuntu 13.04
Add rust-alert to the tree and use it to allow a new URL to be navigated to
Refactor compositing a bit to conform to modern Rust style
Implement the beginnings of scrolling via dragging.

There are tearing problems and GLUT does not support the mouse wheel.
Still, it's a start!
auto merge of #460 : pcwalton/servo/scrolling, r=metajack

r? @metajack
Update rust-mozjs as change of JSVal
Fix codejen as JSVal changed 64 -> {64}