Added option to sip_settings to allow using any configuration file

Ignore-this: 7e5575dc8e46335e8e92a13192104092

darcs-hash:20090723102033-1e3d8-b8551e0f155bd59193aba4e0a25769258fa7f654
Added hold and unhold methods to IMediaStream

Ignore-this: 57bc22d7d76b1f2a7bb22997e56cbc70

darcs-hash:20090722152229-1e3d8-dd7c2f2b93eecf483d6550e00918a2fee0b334cc