Add rust-png submodule.
Enable `-o` option for output_file.
Bump rust-azure pointer.
Clean up check targets in Makefiles.
Write out PNG file after compositing based on `-o` option.
Update README with new deps.
Update rust-png to support libpng 1.2.
auto merge of #590 : metajack/servo/png-output, r=metajack,me

This enables the `-o FILE` command line option to render the window contents to a PNG after rendering and exit Servo.
Update remove_tile, add automatic tile removal, fix tests for quadtree
auto merge of #591 : eschweic/servo/del-tiles, r=pcwalton

The quadtree is now initialized with a memory limit, and tiles are deleted automatically if that limit is exceeded. The memory limit can also be set to None to prevent this behavior.