Add support for function, date & null props

This includes:

@wulftone's older fix for function props, mentioned here: https://gist.github.com/kurtmilam/1868955#comment-767261
@kappostin's older fix for null props, mentioned here: https://gist.github.com/kurtmilam/1868955#comment-810681
And the fix I added for date props today, as a response to @tkalfigo's comment, here: https://gist.github.com/kurtmilam/1868955#comment-1240838

There's a working Fiddle with these fixes and a test of the date part here: http://jsfiddle.net/texinwien/8tcLf/2/