FIX: Fix move button number

Closes #250.

Can be tested manually with this:
```Python
from vispy.app import Canvas

c = Canvas(close_keys='escape')

@c.events.mouse_move.connect
def on_move(event):
    print('move:  %s' % event.button)


@c.events.mouse_press.connect
def on_press(event):
    print('press: %s' % event.button)
    
c.show()
c.app.run()
```

Ready for review/merge, @rossant this fixes an issue you brought up.
Sometimes previously the `button` was being set to `0`. I assume this is what was originally meant by `can be None` -- or are there times it will actually be `None`?