Material Switching Extruder support

I would have wait to have this more perfectioned and experimented, and maybe some awesome achievement prints before releasing it; but I managed to make stable within the firmware, and seems to have potential so I'm gonna do it now.

==Description==
This commit adds support for some fake dual extrusion, or material switching extrusion if you prefer, that allows you to extrude 2 filaments with a single nozzle, with the help of a "Y" joint at the entrance of the nozzle.
The Gcode alterations that allow you to make this without incidents are up to you, with your slicer.

==How it works==
It basically wires nozzle 1 and nozzle 2 together while the steppers are separate.
-The printer recognizes and updates only one nozzle, commands like M104 and M140 update nozzle one only.
-Yet, it recognizes two extruders, so T0 and T1 will activate one stepper or another. 

==Current Limitations==
-There is support for only one nozzle feeded by only 2 extruders
-This mode is not compatible with redundant thermistors (I deactivated it, because I couldn't test it)

=How I tested it=
The mode has been tested successfully on a RAMPS 1.3, with 2 jonaskuehling bowden extruders, feeding an E3D nozzle with a thermocouple.
I used a PTFE tubing made "Y" joint to feed the nozzle with the filament from the two extruders (http://www.thingiverse.com/thing:330757).