Interruption, time-out, and deterministic time-out ("resource-out") features.

Interruption, time-out, and deterministic time-out ("resource-out") features.
Details here: http://goedel.cims.nyu.edu/wiki/Meeting_Minutes_-_October_14,_2011#Resource.2Ftime_limiting_API
This will need more work, but it's a start.

Also implemented TheoryEngine::properPropagation().

Other minor things.