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.
> won't actually get properly removed by this code...

I do not get why not...  I think the last changes from @kdodia do the job...