Fork with RPN calculator

Thanks for making jQuery.NumPad.  FYI, I forked it and added a rudimentary RPN calculator with a few functions - https://github.com/MitchBradley/jQuery.NumPad .

I hesitate to issue a pull request because

- I haven't added hooks for internationalizing the new button names
- Most people would probably prefer an infix calculator

But perhaps some people might find it useful.
