Add zoom_at_point() method to enable.Viewport

[jwiggins]

Currently the logic for handling zoom in/out at a point is in ViewportZoomTool.normal_mouse_wheel. This is not the best place for it, because it means that one cannot programmatically zoom in to a viewport without attaching this tool and calling the method on the tool. Since the viewport supports zoom, and since doing a "proper" zoom (setting new view bounds, etc.) always requires this logic, this code should be split out into a zoom_at_point() method on the Viewport.

https://svn.enthought.com/enthought/ticket/1294