Made local_ip setting nillable to allow listening on any address

Ignore-this: 412fb82c62ae531c38c43453f30cc37d

darcs-hash:20090708073513-1e3d8-2abafa772110282b9d6511d7a702e389625068a5
Added SIPCoreInvalidStateError core error

Ignore-this: 1027fbc39e646d3e66d8d66064c02549

darcs-hash:20090708073609-1e3d8-e2820269001df2a60e0a7c571dae25e307a8640d