Add support for Chart.js

As discussed on issue #21, added a initial suppor for Chart.js.

https://github.com/ankane/chartkick/issues/21

I dont really know javascript, tried to do the best, but i found hard to automate color generation for the charts, and to work with the legends, as Chart.js doesn't support them.

I made it in 2 commit, adding the Chart.js on the second, as you probably wont want to add the library to chartkick.

Hope you like it.