Configuration.h, pins.h: Add MOTHERBOARD 84 for Teensy++2.0

Add a MOTHERBOARD 84 configuration for the Teensy++2.0 breadboard that was the basis for the Teensylu and the Printrboard.  

An example of this hardward in use it is at http://reprap.org/wiki/Teensy_Breadboard

Marlin compiles nicely on the command line with:
   DEFINES=AT90USBxx_TEENSYPP_ASSIGNMENTS HARDWARE_MOTHERBOARD=84  make