Add web-platform-tests submodule and test runner
auto merge of #2089 : Manishearth/servo/wpt, r=Ms2ger

See #1479

This sets up a basic environment for running wpt. It instantiates a virtualenv, generates the manifest if it doesn't exist*, and runs the tests.

Currently @Ms2ger's [`run-in-servo` fork of wpt](https://github.com/Ms2ger/web-platform-tests/tree/run-in-servo/)  is used.

Note: I did everything in a shell script instead of make since one cannot run `source` from a makefile (the environment is immutable, apparently)


*It does not, however, update the manifest. We'll have to add an extra script for that.