Fix zoom mode when drag_button set to None.

When the user set `drag_button=None` on the `ZoomTool` and entered zoom mode (by pressing 'z'), the left-button would start the zoom-box, but releasing the left-button did not end the selection.

Try running [edit_line.py](https://github.com/enthought/chaco/blob/master/examples/demo/edit_line.py), pressing 'z', then pressing the left-mouse-button, and releasing. (This assumes PR #93 hasn't been merged, since it removes the `drag_button=None` line.)

This might not be the right fix (depending on the intended behavior). Alternatively, `normal_key_pressed` should check for `drag_button == None`, so that the left-mouse-button doesn't start the 'pre_selecting' mode.