Added a script for Mac OS X that builds a shared library (.dylib).

I've created a little script to cmake, make and install glfw 3 on a Mac OS X system. This script makes it a lot easier and faster for me to rebuild everything after an update of the glfw repository and I want to share it with the community.

Simply put the script into your glfw 3 root directory and run it - it determines the current working directory automatically.
Because the script deletes the cmake cache file and installs die dylib into the /usr/local/ directory, it relies on sudo calls, thus on entering the admin password.