don’t use reversed identifiers for include guards (Thanks Markus)

Done with:

    sed -in 's/\(ifndef\|define\) _\([0-9A-Z_]*\)$/\1 I3_\2/' include/**/*.h

fixes #804

include/commands_parser.h

@@ -7,8 +7,8 @@
  * commands.c: all command functions (see commands_parser.c)
  *
  */
-#ifndef _COMMANDS_PARSER_H
-#define _COMMANDS_PARSER_H
+#ifndef I3_COMMANDS_PARSER_H
+#define I3_COMMANDS_PARSER_H
 
 #include <yajl/yajl_gen.h>