Added sip_status_messages dictionary-like object to sipsimple.core

darcs-hash:20090417125027-58103-ce165a27e495ad0c6df208b1311508975f75c694
Used sipsimple.core.sip_status_messages in sip_audio_session.py

darcs-hash:20090417125247-58103-c3deea375faa2468606402f697ef5acf7de32f56