trial should include Python stdlib's logging output in its own log

[itamar]

For trial users who use Python stdlib logging, there's no good way to ensure that those logs end up anywhere useful, without doing something like adding a log handler in the test package's __init__.py or other hacks. Trial should probably just include these logs in its own log file by default, or as command-line option, since it's a stdlib library and in common use (and moreover we provide an adapter for own logging).