Don't bundle create-react-class & allow Formsy to be used globally

[vonkruel]

Hi,

When I try to load Formsy-0.19.5 with a <script> tag, the page dies with this error:

create-react-class could not find the React object. If you are using script tags, make sure that React is being loaded before create-react-class.

This error happens even though React has been loaded with a <script> tag before loading Formsy with another <script> tag. If we treat create-react-class as external like react itself, the problem disappears. Is this acceptable?