Add detection for page loads.
Add exit_after_load flag to options, and make them cloneable.
Update rust-azure pointer to get cloneable BackendType.
Add finished callback to windowing, and implement exit after load.
auto merge of #552 : metajack/servo/detect-page-loaded, r=pcwalton

This doesn't do anything, but adds the stuff to detect the page load. This will be needed to write out the composited texture to PNGs for reftests.

r? @sfowler