Added support for redirect

As requested in https://github.com/gin-gonic/gin/issues/84 (I also wanted it for the P-R-G pattern). I think this is the most flexible solution.

We could offer a Redirect(location string) which defaults to 302 and a RedirectCode(location string, code int). What do you think? I think people should be aware of what status code they are returning so the implemented solution is enough for me.