Legend follows highlighted points

This pull request adds the ability to make legend follow highlighted points

![optimized](https://cloud.githubusercontent.com/assets/290451/2844508/4e3f2098-d08b-11e3-8d55-cc10fd30c59a.gif)

I realize this is a fairly standard behavior amongst other charting libraries, but it's never one I've liked. For example, on the `customLabelFollow` test page that you've added, there are wide open spaces to the right of the chart. The label flipping is purely artificial.

Would there be fundamental problems with checking if the legend `<div>` fits in the _window_, rather than the chart `<div>`?