Rename deref_aliases back to ldap_deref_aliases

The "ldap_deref_aliases" option has accidentally been renamed to
"deref_aliases".  Revert that change (but accept both names for a
while), so that the option name now matches the documentation again.
Merge pull request #302 from weiss/ldap-deref-aliases

Rename deref_aliases back to ldap_deref_aliases
Remove invisible presence fields from c2s #state

Invisible presence isn't supported anymore, so the corresponding
ejabberd_c2s #state fields were unused.
Merge pull request #307 from weiss/remove-invisible-presence

Remove invisible presence fields from c2s #state
Add new hook: c2s_post_auth_features

The c2s_post_auth_features hook can be used to extend the list of stream
features offered after authentication.
Offer CSI stream feature only if configured

Don't offer the CSI stream feature when mod_client_state isn't actually
configured to filter stanzas.  This makes sure clients won't send CSI
tags that end up being ignored.