Working version of multiple extruders (up to 3)

Hi guys,

I'm working on adding another extruder to my Prusa. I haven't found suitable firmware w/ multiple extruder support, so grabbed Marlin and made the changes (saw a commit about that, so assume you are interested in multiple extruder support too). There are a lot of changes happening and merges will quickly become difficult, so don't want to hold on sending the pull request even though I havent' spend much time testing. 

The changes should allow support up to 3 extruders. I have RAMPS 1.4, so the config is chnaged to show how to use it w/ 2 extruders (fan moved to pin 4).

There are quite a bit of changes. I've just merged them on top of Bernhard last commit.

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- The temperature control is pretty much complete
  (not sure what to do w/ autotemp though)
  Changed the pins assignment to clearly separate bed and extruder heaters
  and temp sensors, changed a bit how termistor tables are handled.
- The steppers control is rudimentary
  (only chanages what pins it uses depending on the active_extruder var,
   but that's enough for switching extruder in the start.gcode in the
   the profiles)
- Tested only w/ RAMPS 1.4