Add decent API documentation

[mathysp]

This library currently has really limited and outdated documentation. It would be helpful if the author could expand on this documentation.

F.E. the library seems to expect & class ".stepy-finish" on a submit button, but the demo does not reflect this.

[wbotelhos]

My blog is off and i kind of busy to setup a new machine for that.
You could use the demo inside de zip file.