Added Material Switching support for 2 extruders

Added the FAKEDUAL mode, that allows the printer to use two extruders
feeding one nozzle.

Gcodes T0 and T1 select the extruder to use, but commands like M104 and
M140 always heat the first nozzle.

This is not scalable yet, and is uncompatible with redundant
thermistors.

Tested with sucess with a RAMPS. (Motherboard 33)