Refactor Scatter/Bubble Series

Creates a PointSeries as a common base class between points and bubbles.
*responding to comments above*

I think this makes sense for BubbleSeries and ScatterSeries as PointSeries is semi-private, but my experience here is that any public base-classes should contain their API so that the subclasses don't have to copy it. In that regard, I don't think that we should use @extend Series for everything, just @extend PointSeries.