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.
This is possible in principal, I am sure, but once the tools are created with a `plot=plot` keyword arg it seems more difficult to remove them from the serialization. I think we should address improving tool/renderer removal in one go, so this seems ok for now.