New stepper algorithm. Optimized planner.

- Brand-new stepper algorithm. Based on the Pramod Ranade inverse time
algorithm, but modified to ensure step events are exact. Currently
limited to about 15kHz step rates, much more to be done to enable 30kHz
again.

- Removed Timer1. Stepper algorithm now uses Timer0 and Timer2.

- Much improved step generation during accelerations. Smoother. Allows
much higher accelerations (and speeds) than before on the same machine.

- Cleaner algorithm that is more easily portable to other CPU types.

- Streamlined planner calculations. Removed accelerate_until and
final_rate variables from block buffer since the new stepper algorithm
is that much more accurate.

- Improved planner efficiency by about 15-20% during worst case
scenarios (arcs).

- New config.h options to tune new stepper algorithm.

nuts_bolts.h

@@ -39,6 +39,8 @@
 #define MM_PER_INCH (25.40)
 #define INCH_PER_MM (0.0393701)
 
+#define TICKS_PER_MICROSECOND (F_CPU/1000000)
+
 // Useful macros
 #define clear_vector(a) memset(a, 0, sizeof(a))
 #define clear_vector_float(a) memset(a, 0.0, sizeof(float)*N_AXIS)