Add support for function, date & null props

This includes:
@wulftone's fix for function props, [mentioned here](https://gist.github.com/kurtmilam/1868955#comment-767261) 
@kapooostin's 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 result of @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/).