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
This will handle zoomFactor == 0 by calling resetZoom (no effect when zoomed out). Comments?