Adapted sip_register to using SIPApplication and added improvements

Ignore-this: e04c92465a7b79a6e42ae0e30af5cc46

darcs-hash:20090814140633-1e3d8-063bc41e5db5cc00b3512bcb690a30317e6e72f4
Added account.chat.server setting

darcs-hash:20090816101036-58252-45e60e3f00050050e168d733269b1628d8ad6b7c