Display puller RPM, Direct control of puller RPM, other changes.

Changed manual control so that the puller motor rpm is changed by
turning the knob on the LCD.  Extruder RPM controlled via menu.  Added
some default defines for max and min extruder RPM and Puller motor
mm/sec.  Allow extruded length to be zeroed with other stats.  Display
puller RPM.  Fixed issues with PID for puller speed control.  Still has
issues though.  Added way to set PID factors for puller speed control.