Update documentation for include_ip

Add a note that using the include_ip functionality with IPv6 is not a
good idea due to the users network expiring IPv6 addresses quickly.

See preferred lifetime/valid lifetime for routers doing SLAAC in IPv6
for more information.
Add test for AuthTktCookieHelper for IPv6 compat

Add an extra test that sets environ['REMOTE_ADDR'] to an IPv6 address
"::1".
Add tests for AuthTicket/parse_ticket for IPv6 compat

Add some simple tests that test IPv6 compatibility with the AuthTicket
class and the parse_ticket function.
Fix IPv6 compatibility in calculate_digest()

This is a quick fix for adding IPv6 compatibility so that if the
AuthTktAuthenticationPolicy is used with include_ip enabled then it
won't throw an error.
Sign CONTRIBUTORS.txt