Multiwindow using glfw

This branch uses glfw as the window system and ditches glut. But ofAppGlutWindow can still be used by modifying main.cpp

to use glfw:
````c++
ofSetupOpenGL(ofGetWindowManager(), 640,480, OF_WINDOW);	
````

to use glut:
````c++
ofAppGlutWindow window;
ofSetupOpenGL(&window, 1024,768, OF_WINDOW);
````

I was using this branch for the last couple of weeks on ubuntu and think it is pretty stable. But it needs compiled libs for mac & windows. The glfw version I use is a fork of the official glfw repo https://github.com/underdoeg/glfw

We should not merge this before those compiled libs are in there.

I send this pull request mainly to have a place to get a discussion going. It'd be great if someone could try this and test if it works for them too :)