Add support for Django 1.5

Support for Django 1.4 is maintained.

I have run tests, automated and manual agains both version 1.4 and 1.5, nothing unsual observed. I would love if someone would give this patch a whirl.

One un-expected change is switching the cookie mechanism from using file based cookies to cryptographically signed session cookies. To be frank there is no good reason for doing this other than the unit tests fail when using the file based cookie store.

This affects issues #407, #415 and #416