[Bug 865378] Add support for multiple Y axises.

This uses "axis groups". Each axis group's maximum is calculated, and then the entire group is scaled so that the maximum is 1.0. Then the axises and the hover tool tips are adjusted to show the correct numbers. The scaling is adjusted as the zoom is changed and as axises are toggled on and off. I updated the question stats graph to make use of these features

* `initData` and `makeSeries` modified to calculate scales for each axis group, and use them when drawing.  
* Wrote a custom y axis class that makes room for itself around the graph, inserts itself, and supports being added multiple times. It also sets its scale based on a parameter instead of of data (since the data is just from 0 to 1 now).
*  Added a new Hover class, making it respect the scale of series as well, so that the hover tool tips show the original values. I also made the BarHover class an extension of this, for DRYness.

r?

> This is kind of hacky, and the more I do this, the more I want to just use d3 directly.