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.
What if a users passes in a `PanTool` by object reference? e.g. `Pantool(dimensions=["width"])` instead of `"pan"`