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.
If we really wanted to be fancy 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.