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

config.h

@@ -198,6 +198,12 @@
 // case, please report any successes to grbl administrators!
 // #define ENABLE_XONXOFF // Default disabled. Uncomment to enable.
 
+// Uncomment the following define if you are using hardware that drives high when your limits
+// are reached. You will need to ensure that you have appropriate pull-down resistors on the
+// limit switch input pins, or that your hardware drives the pins low when they are open (non-
+// triggered).
+// #define LIMIT_SWITCHES_ACTIVE_HIGH
+
 // ---------------------------------------------------------------------------------------
 
 // TODO: Install compile-time option to send numeric status codes rather than strings.