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)

That was probably some copy and paste from either [tests/customLabel.html](https://github.com/danvk/dygraphs/blob/master/tests/customLabel.html#L44) or [tests/customLabelCss3.html](https://github.com/danvk/dygraphs/blob/master/tests/customLabelCss3.html#L19)