Adding support for band.invert()

[bognikol]

Band scale maps an array of discrete values to an array of bands which occupy whole range (padding can be treated as a band). As each point in range can be mapped to unique value from domain, invert function for band scale can be implemented.

I need this change for react-vis. Zoom for plots with ordinal values on X axe doesn't work.

[mbostock]

Related #64.