Cleaned-up limit pin reporting and comments.

- Cleaned up the limit pin state reporting option to display only the
state per axis, rather than the whole port. It’s organized by an XYZ
order, 0(low)-1(high), and generally looks like `Lim:001`.

- Separated the control pin state reporting from limit state reporting
as a new compile option. This stayed the same in terms of showing the
entire port in binary, since it’s not anticipated that this will be
used much, if at all.

- Updated some of the gcode source comments regarding supported g-codes.

grbl/grbl.h

@@ -23,7 +23,7 @@
 
 // Grbl versioning system
 #define GRBL_VERSION "0.9i"
-#define GRBL_VERSION_BUILD "20150307"
+#define GRBL_VERSION_BUILD "20150314"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>