Added restore settings defaults command.

- New restore setting defaults command. Only wipes ‘$$’ setting in
EEPROM and reloads them based on the defaults used when Grbl was
compiled. Used with a `$RST` command

NOTE: `$RST` is intentionally not listed in the Grbl ‘$’ help message.

doc/log/commit_log_v0.9i.txt

@@ -1,3 +1,12 @@
+----------------
+Date: 2015-05-29
+Author: Sonny Jeon
+Subject: Added G61 exact path support.
+
+- G61 exact path is the Grbl default path control mode, so it’s now
+added as a supported g-code.
+
+
 ----------------
 Date: 2015-05-27
 Author: Sonny Jeon