Make the package go-gettable.

A lot of discussion took place in #82, lets move it to here now.

@shurcooL can you verify if the update of the GLFW sources (to the revision mentioned in the changelog) https://github.com/slimsag/glfw3/commit/0b950381db57c564286edd6caeee50fe3f2b28dc still works on OS X?

I can confirm that building on windows/amd64 and linux/amd64 works A-OK.

I do not have a freebsd machine, and I also do not have a machine with Wayland installed (`go get -tags "wayland" github.com/slimsag/glfw3`).

Does anyone have comments/concerns/etc regarding this proposal?

Note that this includes @shurcooL's GLFW 3.1 changes.