Fix redirect handling and add reverse_urls support

@michaeljoseph The work you did led me to discover that djproxy was not handling downstream redirects correctly. requests, by default, follows redirects. If djproxy was proxying something that returned one, it would dutifully follow that redirect, fetch the results, and send that to the requestor. That was really, really wrong, but it had the ever so slight advantage of not requiring anything like `ProxyPassReverse`.

This pull fixes djproxy's handling of redirects and it introduces ProxyPassReverse-like functionality.

In relation to the changes you made: note that generate_routes will behave erroneously if prefixes do not have leading slashes (this is contrary to the way we originally documented the functionality).

Also, this fixes #3