Fix code generated for classes starting with "\^"

Before this commit, incorrect regexps were produced for classes starting
with "\^". For example, this grammar:

  start = [\^a]

didn't match "a" because the generated regexp inside the parser was
/^[^a]/, not /^[\^a]/ as it should be.

This commit fixes the issue by escaping "^" in |quoteForRegexpClass|.

Fixes GH-125.

lib/utils.js

@@ -183,6 +183,7 @@ var utils = {
       .replace(/\\/g, '\\\\')  // backslash
       .replace(/\//g, '\\/')   // closing slash
       .replace(/\]/g, '\\]')   // closing bracket
+      .replace(/^/g,  '\\^')   // caret
       .replace(/-/g,  '\\-')   // dash
       .replace(/\0/g, '\\0')   // null
       .replace(/\t/g, '\\t')   // horizontal tab