refactored handling of settings '$' command out of gcode module and into settings module

protocol.h

@@ -20,6 +20,12 @@
 #ifndef serial_h
 #define serial_h
 
+#define STATUS_OK 0
+#define STATUS_BAD_NUMBER_FORMAT 1
+#define STATUS_EXPECTED_COMMAND_LETTER 2
+#define STATUS_UNSUPPORTED_STATEMENT 3
+#define STATUS_FLOATING_POINT_ERROR 4
+
 // Initialize the serial protocol
 void protocol_init();
 
@@ -27,4 +33,7 @@ void protocol_init();
 // come in. Blocks until the serial buffer is emptied. 
 void protocol_process();
 
+// Executes one line of input according to protocol
+uint8_t protocol_execute_line(char *line);
+
 #endif