Rename 'PITHOS_AUTHENTICATION_URL' setting to 'ASTAKOS_URL'. Also remove
'PITHOS_AUTHENTICATION_USERS' from docs (it was removed from code in
commit 8df4fae).

Modify 'ASTAKOS_URL' to point to the node hosting the Astakos service
instead of im endpoint, i.e. rename from
'accounts.example.org/im/authenticate' to 'accounts.example.org'.