Refactor Scatter/Bubble Series

Creates a PointSeries as a common base class between points and bubbles.
Sorry, I posted this comment on the wrong line before:

Actually, I've tried out adding @extends insight.PointSeries to the BubbleSeries docs and this solves the issue by adding a link to the parent class like this:

![image](https://cloud.githubusercontent.com/assets/4106217/4203158/b8b85e94-382d-11e4-95df-2253d8e2654a.png)

So I don't think this really needs another ticket since it can quickly be added to this PR, including adding @extends insight.Series to each of our series.