Pushed limits active high option. Updated defaults.h. Misc bug fixes. Cleaned up codebase.

- Pushed limit switch active high option (i.e. NC switches).

- Updated defaults.h to be in-line with the new settings.

- Refactored feed hold handling and step segment buffer to be more
generalized in effort to make adding feedrate overrides easier in the
future. Also made it a little more clean.

- Fixed G18 plane select issue. Now ZX-plane, rather than XZ-plane, per
right hand rule.

- Cleaned some of the system settings by more accurately renaming some
of the variables and removing old obsolete ones.

- Declared serial.c rx_buffer_tail to be volatile. No effect, since
avr-gcc automatically does this during compilation. Helps with porting
when using other compilers.

- Updated version number to v0.9b.

- Updates to README.md

planner.h

@@ -28,25 +28,24 @@
   #define BLOCK_BUFFER_SIZE 18
 #endif
 
-// This struct is used when buffering the setup for each linear movement "nominal" values are as specified in 
-// the source g-code and may never actually be reached if acceleration management is active.
+// This struct stores a linear movement of a g-code block motion with its critical "nominal" values
+// are as specified in the source g-code. 
 typedef struct {
-
   // Fields used by the bresenham algorithm for tracing the line
-  // NOTE: Do not change any of these values once set. The stepper algorithm uses them to execute the block correctly.
-  uint8_t direction_bits;            // The direction bit set for this block (refers to *_DIRECTION_BIT in config.h)
-  int32_t steps[N_AXIS];            // Step count along each axis
-  int32_t step_event_count;         // The maximum step axis count and number of steps required to complete this block. 
+  // NOTE: Used by stepper algorithm to execute the block correctly. Do not alter these values.
+  uint8_t direction_bits;    // The direction bit set for this block (refers to *_DIRECTION_BIT in config.h)
+  int32_t steps[N_AXIS];     // Step count along each axis
+  int32_t step_event_count;  // The maximum step axis count and number of steps required to complete this block. 
 
   // Fields used by the motion planner to manage acceleration
-  float entry_speed_sqr;             // The current planned entry speed at block junction in (mm/min)^2
-  float max_entry_speed_sqr;         // Maximum allowable entry speed based on the minimum of junction limit and 
-                                     //   neighboring nominal speeds with overrides in (mm/min)^2
-  float max_junction_speed_sqr;      // Junction entry speed limit based on direction vectors in (mm/min)^2
-  float nominal_speed_sqr;           // Axis-limit adjusted nominal speed for this block in (mm/min)^2
-  float acceleration;                // Axis-limit adjusted line acceleration in (mm/min^2)
-  float millimeters;                 // The remaining distance for this block to be executed in (mm)
-  // uint8_t max_override;           // Maximum override value based on axis speed limits
+  float entry_speed_sqr;         // The current planned entry speed at block junction in (mm/min)^2
+  float max_entry_speed_sqr;     // Maximum allowable entry speed based on the minimum of junction limit and 
+                                 //   neighboring nominal speeds with overrides in (mm/min)^2
+  float max_junction_speed_sqr;  // Junction entry speed limit based on direction vectors in (mm/min)^2
+  float nominal_speed_sqr;       // Axis-limit adjusted nominal speed for this block in (mm/min)^2
+  float acceleration;            // Axis-limit adjusted line acceleration in (mm/min^2)
+  float millimeters;             // The remaining distance for this block to be executed in (mm)
+  // uint8_t max_override;       // Maximum override value based on axis speed limits
 } plan_block_t;