Corral debug command-line options into a --debug/-Z flag

This should make help output a lot cleaner and simplify the way that
uncommon debug options are passed.
auto merge of #3879 : mrobinson/servo/debug-options, r=cgaebel

This should make help output a lot cleaner and simplify the way that
uncommon debug options are passed.
Invert control flow, fix resizing, and improve checkerboarding
significantly by giving tiles some time to paint before we render
unrendered content.
auto merge of #3761 : pcwalton/servo/smooth-scrolling, r=larsbergstrom

r? @glennw @mbrubeck
Update web-platform-tests.
auto merge of #3891 : Ms2ger/servo/wpt_20141103, r=jdm