added scrollwheel support for GLFW

implemented scroll events based on the already provided scroll_cb in
ofAppGLFWWindow.cpp

feature tested on osx 10.8.5, both with apples magic mouse and a standard scrollwheel mouse.

usage in testApp:
void testApp::mouseScrolled(double x, double y){
    ofLog(OF_LOG_NOTICE, "scroll X:" + ofToString(x));
    ofLog(OF_LOG_NOTICE, "scroll Y:" + ofToString(y));
}