Some tidying.  Increased SD delay to 20ms.
Merge remote-tracking branch 'dc42/duet' into duet
Merged David Crocker's network and A->D code improvements from https://github.com/dc42/RepRapFirmware/tree/duet
Applied 3D-ES and bobc's fixes to the ftoa() function (see http://forums.reprap.org/read.php?340,293724).
Removed legacy support for Duet v0.4 heaters.  Last commit before changes to max velocity code.
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.