Added option to PersistentTones for playing after the specified interval

Ignore-this: da7107e876275077c7acc4af785162d4

darcs-hash:20090828110210-1e3d8-ce52bf4ab3febc83d61f009841ba616ca2fe1cfc
Modified Session to not require an SDP version increase in reINVITEs

Ignore-this: 88efc31406ea35881022930caab9ca4a

darcs-hash:20090828122426-1e3d8-3db2a9fbb6406a52da4ee387c0997d6bab8b3b9b