clients.config: do not replace twisted.log's defaultObserver

darcs-hash:20090325093339-b2873-0aa90e54cf1ddd8eaa26837ac7cbb27df50ba4f9
green.core: make GreenEngine a singleton

darcs-hash:20090325093424-b2873-5a5b70a1ef7e3669fa35bb78363ce6fbc7498965