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