Code added to add a bed plane z = aX.x + aY.y + aC to all movements.
Note that this means that X/Y minimum speeds and accelerations now take
precedence over Z, unless there is only Z movement.
Z probing being implemented under G32.  Not finished yet, so don't call G32...
Z probing movements all implemented, but Z values not yet recorded.
Z probing now fully implemented, except for the physical probe itself.  (I.e. it's all working, but is still on the microswitch.)
Z probing now working.  The probe signal connects to AD7 which (confusingly)
is A0 on the Arduino.  The signal seems quite noisy.  I have put a 10uF
capacitor across its supply.  Maybe add an 0.1uF across its signal?
Two values need to be set in Platform.h:

If v is the value from the A->D on A0, then:

z = v*Z_PROBE_GRADIENT + Z_PROBE_CONSTANT (mm)

Z_PROBE_GRADIENT is dependent on the IR proximity device (though they may
be consistent enough all to have the same value) and Z_PROBE_CONSTANT depends
on how high it is mounted relative to the nozzle.

To generate these values, uncomment the bits of code in and around Platform::Spin()
zero the Z axis by eye, run the program, and raise Z by 0.1mm increments.  Put
the raw A->D values in the spreadsheet in the Data directory, and it will
give you the values of Z_PROBE_GRADIENT and Z_PROBE_CONSTANT.
on-board ethernet pin defs removed.
Merge branch 'duet' of https://github.com/reprappro/RepRapFirmware into duet
ignore Release folder
Z probing merged from duet branch.
unused EMAC pin defs commented out.