FWRETRACT in mm/s

Firmware retraction now stores the retract and recover speeds in mm/s
instead of mm/min. This makes it match the units of the maximum
feedrate, and fixes problems with modifying the value via LCD control
panel. From gcode, the values are still taken in mm/min to match the
units of G1 and similar, and they are converted to mm/s before they are
stored.

I also lowered the default retract feedrate to make it less likely to
cause problems for geared extruders when the user hasn’t bothered to
set a reasonable maximum feedrate, though users should be setting both
of these values to suit their hardware.
thermistortables.h: Add comments that table 60 uses beta=3950.
fix bug in M200 with multiple extruders
fix bug in M200 with multiple extruders
fix bug for dual extruders not working

some guy find that marlin not working good for dual extruders delta .
when type T0 or T1 to active extruder and  E0 or E1  move causing XYZ
motion . so i locales the bugs and fix it , I have dry run the fix.
Merge pull request #864 from whosawhatsis/M200

fix bug in M200 with multiple extruders
Merge pull request #866 from sakunamary/Marlin_v1

fix bug for dual extruders not working
Merge pull request #860 from drf5n/therm3950

thermistortables.h: Add comments that table 60 uses beta=3950.
Change CONTROLLERFAN_PIN for board 35 to -1, as it would conflict with board's 33 extruder pin, causing burnout (it's the same board, just different number of fans/extruders, therefore should be pin-compatible)
Merge pull request #837 from whosawhatsis/fwretract

FWRETRACT in mm/s
Update Readme descriptions for M200, M207 and M208
Merge pull request #871 from whosawhatsis/fwretract

Update Readme descriptions for M200, M207 and M208
Merge branch 'Marlin_v1' into Marlin_v1_Mendel90