Fix for slic3r 1.1.1

Changed code that looks for G1 Z commands to find object height, so that
it ignores G1 Z commands in the end-of-file comments added by slic3r
1.1.1
Increased max number of open files
Work around SDHC stack bug

Reduced block read size when retrieving file info to work around an
intermittent bug that causes the SDHC card/ FATfs stack to return data
from the wrong sector sometimes. Tidied up some other code.
Delete RepRapFirmware-058b-dc42.bin
Delete RepRapFirmware-058a-dc42.bin
Merge pull request #1 from zombiepantslol/duet

Added new "I" parameter to M106
Fixed nested file bug, other minor changes

Fixed a bug when one file was called from another
M111 S2 command now displays time since last reset
M106 'I' parameter now only inverts fan PWM when >0
Non-homing moves when bed probing are now done at max speed
M999 reset command now delays 0.5 second to allow response to be sent
back to browser, otherwise it may retry the reset