Add glfw and glfw-rs to servo build.
Fix clean targets.
Replace glut with glfw.

Also fixes initial window size to get based on the windowing code instead of
being hardcoded everywhere. This code works on HiDPI displays, but does not
appropriately scale the output up yet.
auto merge of #563 : metajack/servo/glfw, r=jdm

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