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)

dygraphs generally follows the [Google JS Style Guide](https://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml), which says to put a space between `if` and the `(` (also below).