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)

Please move these into a `<style>` block -- the legend gets a standard class, `dygraph-legend`. I'd like to deprecate `labelsDivStyles` in the future; it's better to set your styles in CSS.