Make Rickshaw.Graph.Legend a real class
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.

zhangweiabc/rickshaw,shutterstock/rickshaw,toniher/rickshaw,IoTboy/rickshaw,pombredanne/rickshaw,butterflyhug/rickshaw,dpobel/rickshaw,zhangweiabc/rickshaw,toniher/rickshaw,butterflyhug/rickshaw,RobertoMalatesta/rickshaw,dpobel/rickshaw,adeelshahid/rickshaw,butterflyhug/rickshaw,egorovpavel/rickshaw,dwt/rickshaw,zhangweiabc/rickshaw,dwt/rickshaw,egorovpavel/rickshaw,IoTboy/rickshaw,boomtrain/rickshaw,boomtrain/rickshaw,shutterstock/rickshaw,dwt/rickshaw,openhose/rickshaw,openhose/rickshaw,toniher/rickshaw,egorovpavel/rickshaw,mcanthony/rickshaw,mcanthony/rickshaw,adeelshahid/rickshaw,JaySunSyn/rickshaw,boomtrain/rickshaw,IoTboy/rickshaw,JaySunSyn/rickshaw,shutterstock/rickshaw,openhose/rickshaw,pombredanne/rickshaw,RobertoMalatesta/rickshaw,mcanthony/rickshaw,JaySunSyn/rickshaw,pombredanne/rickshaw,dpobel/rickshaw,RobertoMalatesta/rickshaw,adeelshahid/rickshaw