Add test case for Legend
Make Rickshaw.Graph.Legend a real class

This makes it possible to extend the legend to add custom styling
or interactivity. This commit should not change behavior or options.
Allow Legend subclasses to override the className
Extract legend rendering into render function that can be called multiple times
Return rendered line from addLine
Merge pull request #339 from dwt/make-legend-a-class

Make legend a class