New EEPROM restore functions.

- Tweaked the previous EEPROM restore implementation and added new
functionality.

- `$RST=$` restores the `$$` grbl settings back to firmware defaults,
which are set when compiled.

- `$RST=#` restores the `$#` parameters in EEPROM. At times it’s useful
to clear these and start over, rather than manually writing each entry.

-`$RST=*` wipe all of the data in EEPROM that Grbl uses and restores
them to defaults. This includes `$$` settings, `$#` parameters, `$N`
startup lines, and `$i` build info string.

NOTE: This doesn’t write zeros throughout the EEPROM. It only writes
where Grbl looks for data. For a complete wipe, please use the Arduino
IDE’s EEPROM clear example.

- Refactored the restore and wipe functions in settings.c to
accommodate the new commands.

grbl/grbl.h

@@ -23,7 +23,7 @@
 
 // Grbl versioning system
 #define GRBL_VERSION "0.9i"
-#define GRBL_VERSION_BUILD "20150618"
+#define GRBL_VERSION_BUILD "20150620"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>