Allow CharacterClassEscape in character classes

Character classes now accept JavaScript-like escapes (CharacterClassEscape),
namely \d, \D, \s, \S, \w, \W as defined in
http://www.ecma-international.org/ecma-262/5.1/#sec-15.10.2.12

This hesitant PR (just wanted to experience debugging PEGjs internals) fixes https://github.com/dmajda/pegjs/issues/247