Made sipsimple.core SDP classes properly subclassable

darcs-hash:20090416162715-58103-a7840926982eeb5b11cb45f3d028f54caa9f52f0
Removed item from TODO

darcs-hash:20090416162750-58103-f3351221f9d084bd20bb7e11ee654e502e55f12f