Improve redirect handling

Hi,

I've added 307 handling (#155) + for 301 and 302, I mimic IE behaviour, to replace HTTP verb by GET after redirection. For technical details: https://github.com/kennethreitz/requests/pull/269

It's a really an issue for us, because we need to interact with some old fashion webapp without WS API that use a lot this assumption.
Moreover, this behaviour is now official, 307 HTTP code has been created to handle correctly redirections.

Thanks for reviewing.

Regards.