stdout/stderr handling in base.command

[uwefladrich]

While addressing #41 and #42 some logging issues have been introduced, namely

1. stderr is completely swallowed if the task completes without error, and
2. stdout is delayed until the task is completed.

A better implementation of base.command logging is needed. It should

• log stdout and stderr with only reasonable delay
• use the existing logging system to log stdout and stderr
• allow to suppress stdout and/or stderr
• allow to save stdout and/or stderr in the context