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.

gcode.h

@@ -1,5 +1,5 @@
 /*
-  gcode.c - rs274/ngc parser.
+  gcode.h - rs274/ngc parser.
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
@@ -33,4 +33,7 @@ uint8_t gc_execute_line(char *line);
 // Set g-code parser position. Input in steps.
 void gc_set_current_position(int32_t x, int32_t y, int32_t z); 
 
+// Clear g-code parser position
+void gc_clear_position();
+
 #endif