Documented special case of objects.extract()

also fixed error messages
Implemented Task.runNestedQuietly()

supresses log of nested task unless an error occurred
PersistentTaskContext uses Q.async syntax
Implemented database access with sqlite support (for now)

The data base parameters can be specified in the config.yaml
Data base connection is shown on the site-overview page
Failed tasks show stacktrace, successful tasks log "Done."

Errors end with "failed: ...", but successful tasks do not necessarily log
a final message.

We don't want this for nested tasks, but it is good for top-level tasks.
test repositories include migrations
Implemented MigrateTask

implicitly tested by UpgradeTask spec, but that just ensure that it does
not fail.