Consistent z homing

Hi Daid,

(I like the new pre-print filament push with the higher bed, by the way.  I was going to send you something like that but you beat me to it.)   ;-)

This branch includes both the #19 Z homing fix as well as Arduino 1.0.1 Makefile updates.  Feel free to let me know if you'd like me to split these into two independent branches and pull requests.

I'm happy to wait a few days for confirmation from someone else that this fixes #19 -- so far I haven't seen anyone else say that they've even built it.

I also wanted to add a comment to Configuration.h clarifying that the UM2 is board type 72, but am waiting for someone to confirm that it is (is it?).

http://umforum.ultimaker.com/index.php?/topic/5935-z-axis-homing-inconsistent-on-um2-workaround-and-patch/
http://umforum.ultimaker.com/index.php?/topic/5933-building-marlin-makefile-updates-for-um2-and-arduino-101/

Steve