Added redirect_history property to HTTPResponse instances

Hey guys, first of all, thank you for your awesome work on urllib3.

One thing I missed though was being able to figure out what were the redirects my request had to jump through before arriving at the final destination, and what was this final destination.

To solve this problem I added a new property to instances of HTTPResponse called `redirect_history`. It contains a list of tuples with each status code and redirect location sent by the servers before the request was completed. I also added some test cases to illustrate this behavior.

Please let me know if you think this useful. I'm open to any changes you might require, just let me know and I'll work on it.