Google will create an entityId with your domain, as described in #9.

Might not be complete, needs check with current G Suite interfaces.
But improves on the current situation.

It can now be used with \SimpleSAML\Auth\Simple, although the old name still works too.

Also tweak wording of artifact idp docs a bit re memcache extension.

Minor format and exception message fixes, plus add information about the different communication channels we have today.