Updated limit/homing routine. Works, but needs more TLC.

- Added acceleration to the homing routine.

- Homing now accounts for different step rates when moving multiple
axes without exceeding acceleration limits.

- Homing now updates all internal positioning variables to machine zero
after completion.

- "Poor-man's" debounce delay added.

- Updated the delay_us() function to perform faster and more accurate
microsecond delays. Previously, the single increments would add
noticeable time drift for larger delays.

- Fix a bug in the stepper.c prescalar calculations that was changed in
the last commit.

- Other minor fixes.

stepper.c

@@ -31,7 +31,6 @@
 #include "nuts_bolts.h"
 #include <avr/interrupt.h>
 #include "planner.h"
-#include "limits.h"
 
 // Some useful constants
 #define TICKS_PER_MICROSECOND (F_CPU/1000000)
@@ -299,7 +298,7 @@ ISR(TIMER1_COMPA_vect)
       plan_discard_current_block();
     }
   }
-  out_bits ^= settings.invert_mask;  // Apply stepper invert mask    
+  out_bits ^= settings.invert_mask;  // Apply step and direction invert mask    
   busy = false;
 }
 
@@ -396,7 +395,7 @@ static uint32_t config_step_timer(uint32_t cycles)
   } else {
     // Okay, that was slower than we actually go. Just set the slowest speed
     ceiling = 0xffff;
-    prescaler = 6;
+    prescaler = 5;
     actual_cycles = 0xffff * 1024;
   }
   // Set prescaler