Add support for className prop

[jonbeller]

Solution for #242

I chose className as the name of the prop to match the convention of how CSS classes are specified on React components. It is also necessary to accept className for compatibility with the library styled-components.

I did not remove the ability to call the prop class, but I updated the documentation and examples to prefer className. Let me know if you want me to add back references to class!

[wwayne]

Good to me, thanks :), will deploy to npm late of today