Add support for HTTP-Redirect assertions.
Add destination to LogoutRequest as it's required by ADFS.
Remove unneeded metadata callback.
Added signature to logout request.
Add support for parsing logout responses.
Fix broken test caused by response format change.
Updated some comments.
Changed signing protocol to RSA-SHA256.

By default, ADFS uses RSA-SHA256 and requires an extra config
step to switch it to RSA-SHA1. Since RSA-SHA256 is more secure
anyway, it makes since to use that exclusively.
Added test for sign_get_request.
Removed identity provider from create metadata function.
Minor fixes and version bump.
Merge pull request #3 from Clever/implement-logout

Implement Logout