Feature/twinx

Support for multiple axes.

![screen shot 2014-09-03 at 11 06 44 am](https://cloud.githubusercontent.com/assets/1078448/4137131/533f729e-3384-11e4-8d3d-b61e8f2a742b.png)
I overlooked `Plot.extra_y_ranges`. So, summing up, `Plot` maps names to ranges and then those ranges are used by name in renderers (is this correct?). If so, my suggestion is to, at minimum, to extend the example and add more ranges, more names and more references to ranges so that the intent is clear (because I'm pretty sure that if it's unclear to me, it will be unclear to users). Still I think (at least after a cursory review) that this approach is possibly wrong, but if I pursue this now I won't make new widgets today (e.g. will this respect events when changes are made to a range).