Add zoomScale, rename doPan to pan, doZoom to zoom

[kurkle]

Resolves #126

I was about to just document setting the min and max + chart.update() method, but that way you lose the ability to reset zoom to previous value. So concluded its better to have that functionality built-in.

• Add zoomScale export and chart instance function
• Add sample usage of zoomScale
• Rename exported doPan to pan and doZoom to zoom for consistency with instance API.
• Add any transition mode support to all of these methods
• Add augmentation of UpdateModeEnum to include 'zoom'