Merge pull request #568 from wolfmanjm/upstreamedge

add M600 as alternative to suspend command
add back the unstepped distance to extuder follow mode, and refactor that code
Make millimeters_of_travel consistent about what it uses to detect 0.0F
Merge pull request #569 from Smoothieware/fix/extruder-regression-unstepped-steps

add back the unstepped distance to exrtuder follow mode, and refactor tha...
put median buffer on stack