Improved network recovery from errors and fixed small temp cal error

Improved network error recovery so that it it possible to reconnect
after some types of network error have occurred
Corrected calculation of temperatures to give more accurate results
towards the limits of the ADC range
Fixed slow printing over USB

Fixed printing over USB and via HTTP so that it uses the same lookahead
mechanism as for printing from SD provided the data arrives fast enough.
Added an incoming buffer for data arriving from USB. NOTE: Pronterface
needs to be patched  to send the data fast enough for this to work well.
Also changed signatures of some member functions to be const-correct
Corrected a speling errror
Added binary file
Fixed thin-layer Z-axis printing problem

Fixed problem whereby when printing in thin layers, the z-axis stepper
would try to move too fast, resulting in missed steps. This was because
the lookahead code was setting incorrect z-move velocities when a Z-move
was preceded by an extruder-only move.
Improve performance of SD card upload via USB

This change approximately doubles the speed of SD-card uploading via USB
to 2.2Mbyte/sec on the test system, provided that Pronterface has been
patched to not wait for an acknowledgement after sending each line of
g-code.
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