Bug 945222 - Add test harness for running W3C web-platform-tests.

This adds a new test harness, and an associated
mach command |mach web-platform-tests|.

At present only desktop firefox is supported. Tests
are run in parallel using multiple firefox instances.

It also adds a copy of the web platform tests and a
(incomplete) script for updating them.
* * *
Bug 945222 - Update wptrunner to use proxy autoconfig and localhost in a way that might work on infrastructure.
* * *
Change test runner manager to be a bit more robust?
Bug 945222 - Update web-platform-tests to revision 67e47f0be62d8f9c11879c12f7e26693cd36b557
Bug 945222 - Update web-platform-tests expected data to revision 67e47f0be62d8f9c11879c12f7e26693cd36b557