Make acceleration a software configurable setting

[ErikDeBruijn]

Currently, the FiveD firmware either has acceleration compiled in, or not. If this is made runtime configurable through an M-Code, this would allow the Java Host and ReplicatorG to be used side by side, the java host making use of acceleration and the integrated Skeinforge just using constant linear movements. A feature to set maximum acceleration and use G0 instead of G1 when the head is traveling is related to this. This would allow much faster motion in between extruded segments creating less ooze and missing material (leading to weak parts) problems.