Minor update for memory savings in ring buffer and fleshed out commenting.

No changes in functionality. Path vectors moved from ring buffer to
local planner static variables to save 3*(BUFFER_SIZE - 1) doubles in
memory. Detailed comments. Really need to stop micro-updating. Should be
the last until a planner optimization (ala Jens Geisler) has been
completed.

planner.h

@@ -36,7 +36,6 @@ typedef struct {
   double speed_x, speed_y, speed_z;   // Nominal mm/minute for each axis
   double nominal_speed;               // The nominal speed for this block in mm/min  
   double millimeters;                 // The total travel of this block in mm
-  double delta_mm[3];                 // XYZ travel components of this block in mm                  
   double entry_speed;                 // Entry speed at previous-current junction 
   double max_entry_speed;             // Maximum allowable entry speed