Updated export regex to not break on single quote. Fixes #191

My original fix for this was to just remove the question mark:
`$html = preg_replace("/'label'(.*?)('.*')/", "'label'$1__($2)", $html);`

But after Googling the problem I found this answer:
http://stackoverflow.com/questions/5695240/php-regex-to-ignore-escaped-quotes-within-quotes

This implementation seems more robust.