Skip pan/zoom tool registration

Closes https://github.com/ContinuumIO/bokeh/issues/464

According to @bryevdv there's no easy way to remove a tool from a plot, so this catches pan and zoom tools added via the `tools` attribute and keeps them from being added to the plot.
> we could allow panning and zooming along one axis if only one of the axes is categorical but I would not rate that a high priority.

Yes, probably not so difficult to add... :wink: