Added GLFW shared library version check. Raise ImportError when the GLFW3 shared library cannot be loaded (e.g. because it wasn't found).
The version checks for the GLFW shared library are now done in subprocesses.
While this might seem a bit ugly (at least the code does), it should be obvious that it is better to load a shared library only once per process, than loading one version of a library after another. At least on Mac OS X, this repeated loading lead to warnings on stderr, which this commit should fix.
Suggestions on how to improve this solution are welcome!