Support for regular expressions with non-capturing subpatterns.

It seems that AutoFixture fails to generate anonymous strings for regular expressions that contain [non-capturing subpatterns](http://php.net/manual/en/regexp.reference.subpatterns.php).

The problem is similar to the one reported in issue #1, where AutoFixture treats certain regular expressions syntax tokens as *literals* during the parsing process, thus generating invalid strings.

This pull request solves the problem by modifying the regular expression parsing algorithm to recognize and skip any non-capturing subpattern expressions `?:` after an opening parenthesis.