Allow variable symbol sizes in ScatterPlot

Move functionality from VariableSizeScatterPlot to parent ScatterPlot class.  Also allows ColormappedScatterPlot to use variable marker sizes, closing #97.

![scatterplot-merge2](https://f.cloud.github.com/assets/499079/190484/6dc438ea-7ef2-11e2-8efa-ed4a60cd6861.png)