Refactored audio support

Ignore-this: baaa91fcef39a9a3dfe71174dac76804

darcs-hash:20100310175220-1e3d8-45c38573dca6ca17e0b3ce5e428dad7f0e77ffec
Don't initialize ICE transport if remote SDP doesn't offer ICE

Ignore-this: 1603c6d9e887fcc8d766f148c874e92b

darcs-hash:20100311120421-56b4d-b0adb194654cdb577752e68c978c6bb9f10f70c6