Added support for mouse wheel. 

Implements scroll wheel functionality in GLFW. Seems a bit too straight forward, not sure why this wasn't already in master.

__Only tested on OS X 10.10__

This enables the following:
```c++
void ofApp::mouseScrolled(float x, float y)
{
	ofLog() << "Scroll " << x << "x" << y << endl;
}
```