Zoom feature

Adds a zoom-to-bounding-box on click feature, adressing issue #25. Also includes a zoomFactor in the options, effectively giving people the option to choose how far the camera zooms. Has a special case for circles, as they do not have a bounding box

Demo here:
http://esphen.com/Open-Source/datamaps/src/examples/bubble-in-center.html
In this face, `self.options` is the global options object, but the `zoom` options live within `geographyConfig`, so currently it's always `0.9`.

I think the `zoom` options should live outside of the `geographyConfig`, since everything will be zoomed in.