Update to current GLFW master.

`rm -rf ./glfw/ && git clone https://github.com/glfw/glfw`
Follow suit of GLFW and move clipboard.go contents into window.go.
Fix darwin build tags to match GLFW CMake defaults.

This fixes Retina support.
Update source GLFW revision in README.

It might be good time to start making a development script that updates
the glfw/ subfolder and writes the GLFW revision into some file, so
that it's not possible to forgot to update it.
Not to mention it was relatively hard to find the issue that described
steps to update. For reference, it was:
https://github.com/go-gl/glfw3/issues/82#issuecomment-53175925
Merge pull request #95 from shurcooL/master

Update devel_glfw3.1 branch to current GLFW and fix Retina support.