resolvers in `resolveXYScale` should recursively traverse `children` tree

[dandelany]

The new code in 0.2.1 is improving but has a significant regression - All of the resolvers in resolveXYScales used to recursively check the component's children to find the necessary props. However @thehindenburg 's new mapOverChildren, while being a nicer abstraction, does not do recursion - it only maps over an element's direct children.

Recursion is necessary IMHO because we want to support the use case of the user having intermediate layer components between XYPlot and their charts, eg.:

<XYPlot>
  <CustomChartWrapper>
    <LineChart />
    <AreaChart />
    <BarChart />
  </CustomChartWrapper>
</XYPlot>

Without recursion, this would never try to resolve anything on the actual chart components, it would stop at CustomChartWrapper. We should improve the general resolution functions in resolveXYScales to handle this case

[dandelany]

Related to #35