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

gcode.c

@@ -140,7 +140,7 @@ uint8_t gc_execute_line(char *line)
           case 4: non_modal_action = NON_MODAL_DWELL; break;
           case 10: non_modal_action = NON_MODAL_SET_COORDINATE_DATA; break;
           case 17: select_plane(X_AXIS, Y_AXIS, Z_AXIS); break;
-          case 18: select_plane(X_AXIS, Z_AXIS, Y_AXIS); break;
+          case 18: select_plane(Z_AXIS, X_AXIS, Y_AXIS); break;
           case 19: select_plane(Y_AXIS, Z_AXIS, X_AXIS); break;
           case 20: gc.inches_mode = true; break;
           case 21: gc.inches_mode = false; break;