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.
Merge pull request #303 from weiss/no-csi-if-unconfigured

Offer CSI stream feature only if mod_client_state is enabled
Let Git ignore the "rel/ejabberd" directory
Merge pull request #310 from weiss/ignore-rel-dir

Let Git ignore the "rel/ejabberd" directory
Make directory creation more robust
Process XML-RPC requests via p1_xml and ejabberd_http
Switch to P1 implementation of XML-RPC
Fix format of an XML-RPC response
Fix list unblocking when Riak is used as a backend
Make sure "starttls_required" can't be bypassed

Don't allow clients to circumvent the "starttls_required" option by
enabling XMPP stream compression.
Merge pull request #316 from weiss/really-require-tls

Make sure "starttls_required" can't be bypassed
Remove "xmlrpc" dependency for "make rel"

The "xmlrpc" library isn't used anymore.
XEP-0198: Set #state.conn field on session resume
Merge pull request #318 from weiss/remove-xmlrpc-dep

Remove "xmlrpc" dependency for "make rel"
Merge pull request #317 from weiss/xep-0198

XEP-0198: Set #state.conn field on session resume