Add a parent_domain option for auth_tkt policy

This change adds a new ``parent_domain`` option to
``AuthTktAuthenticationPolicy`` which sets the authentication cookie as
a wildcard cookie on the parent domain. This
is useful if you have multiple sites sharing the same domain.
Merge branch 'master' of github.com:Pylons/pyramid
Merge branch 'master' of github.com:Pylons/pyramid
Merge branch 'auth-parent-domain' of github.com:wichert/pyramid into wichert-auth-parent-domain
Merge branch 'wichert-auth-parent-domain'