Better C++ regex

Based upon this comment: https://github.com/github/linguist/issues/1626#issuecomment-66371622
Other than intending there to be a new line after the class/namespace check, that looks fine.

@bkeepers it is supposed to be PCRE Regex for "any whitespace that does not create new lines or run vertically" - so space or tab - however, as it appears not to be working per that console match try, try the pattern `/[ \t]/` as this is what it should be equivalent to.