remove 'enabled' attribute from EngineTracer

darcs-hash:20090310092241-b2873-c605ab5b639ef0e328e260d1336c0452ff709ab4
make green.engine and logstate completely independent

- add LoggerManager class
- remove register_observer/unregister_observer methods to loggers
- add start/stop methods to loggers

darcs-hash:20090310092606-b2873-8f4465fff5698650c06be61a84c1414c6a9cd992