auto merge of #563 : metajack/servo/glfw, r=metajack

This code replaces glut with glfw. The motivation here is address the GPU driver bugs on Linux when using multiple `Display *` pointers with shared GL contexts instead of a single common one. GLFW has native access methods which provide access to its `Display *`, which appears to be unique among all the similar toolkits.

Details:
- Adds glfw and glfw-rs to the build
- Removes GLUT code and replaces it with GLFW versions
- Fixes hard coded initial window values
- Fixes clean targets
- Event loop doesn't block on windowing events anymore
Add a submodule for rust-cssparser

Even though this parser is not used yet, having it built
and tested with the rest of Servo helps ensure it does not bitrot.