added support for the reprapworld keypad

hi,

i have spend some hours to learn how to handle shift registers and then to add some functionality  to the keypad buttons they were not supported in any way since it was released. currently not all buttons work because i haven't finished yet my prusa i3 yet because of some missing parts from reprapworld :-) and therefore i have only the y axis which i can play with.
what work:
- button f1 open the menu
- button down move the bed down
- button up move the bed up
- button middle move the y axis to home
what did not work:
- all other buttons but i will add the functionality when i get the missing parts

hopefully this piece of code will find a place in the marlin firmware.

king regards
gregor