Added an account setting 'proxy'.
Note that this is only used for sending. The default <local>;<auto> means let
the receiver try a direct connection first and then the proxy discovered from
the server (if any). If you know you're firewalled you can remove the <local>.
If you want to provide your own proxy try something like
"<local>;JID,HOST,PORT". E.g.
"<local>;proxy.somewhere.org,123.123.123.123,7777".
