Merge pull request #1 from illuminarti/Fix_E_Speed

Fix step rate bug (especially for retraction)

Marlin/stepper.cpp

 static unsigned short acc_step_rate; // needed for deccelaration start point
 static char step_loops;
 static unsigned short OCR1A_nominal;
+static unsigned short step_loops_nominal;
 
 volatile long endstops_trigsteps[3]={0,0,0};
 volatile long endstops_stepsTotal,endstops_stepsDone;
   deceleration_time = 0;
   // step_rate to timer interval
   OCR1A_nominal = calc_timer(current_block->nominal_rate);
+  // make a note of the number of step loops required at nominal speed
+  step_loops_nominal = step_loops;
   acc_step_rate = current_block->initial_rate;
   acceleration_time = calc_timer(acc_step_rate);
   OCR1A = acceleration_time;
     }
     else {
       OCR1A = OCR1A_nominal;
+      // ensure we're running at the correct step rate, even if we just came off an acceleration
+      step_loops = step_loops_nominal;
     }
 
     // If current block is finished, reset pointer