Support for modulated IR sensor

Add code to support sensors that allow the IR output to be modulated, to
reduce the sensitivity to ambient IR. Use M558 P2 command to enable
sensor modulation. Also changed the sensor reading averaging code to
give more consistent z-height seeking.
Started work on improvements to the velocity constraints.  Not fully implemented, but this will mean that the maximum velocity in any movement direction will be the constraint, regardless of the mix of X, Y and Z moves.  This has been tested, and is printing.  But beware funny velocity effects...
Make disconnected thermistor show temp as absolute zero again

In a previous commit I changed the temperature calculation to give more
accurate readings at extreme values. This change meant that a
disconnected thermistor no longer shows as absolute zero. Coupled with
the change in thermistor beta value, this meant that a disconnected
extruder thermistor would show as -28.2C, which is above the usual error
threshold of -30C. This commit introduces a special case for a
disconnected thermistor and restores the reading to absolute zero for
that case.
New binary built from previous committed sources
Merge branch 'duet' of https://github.com/dc42/RepRapFirmware into duet
Merging David Crocker's latest changes and fixes.
First implementation of the optimised maximum speed in any direction code.  This is still buggy.  To print with, use the previous release.
Experimental version of Move.cpp...
Added in David Crocker's improvements (many thanks to him).
Added in iamburney's web interface (many thanks to him).
Updated reprap.htm
This commit DOESN'T WORK.  Use the next one or the previous one.  This is an intermediate commit done merely to facilitate the moving of files between machines.
Merge branch 'duet' of https://github.com/reprappro/RepRapFirmware into duet
Tidying small errors introduced (deliberately) at the last commit.  This one should work.
Now that a number of people are working on this code, I have been going
through making the comments more copious, especially in the .h files.
W.I.P.
Continuing with adding comments to .h files
Merge pull request #1 from reprappro/duet

Duet