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.
OK there are actually some wider issues with the tool interface. It's a bit much to go into in an issue right now. The upshot is that passing in tools as objects is actually an impossible chicken and egg problem right now (because they take the plot they are being added to as keyword parameter on construction). That being the case @kdodia is going to refer the PR to just check the strings for now. We need to make some fixes to our tool code but that is a wider issue.