Refactor Scatter/Bubble Series

Creates a PointSeries as a common base class between points and bubbles.
I just don't think this is at all clear:

![image](https://cloud.githubusercontent.com/assets/4106217/4203437/b785e3a0-382f-11e4-9eae-d7011cf3c6b8.png)

And it would be far clearer with the addition of "Gets ...."