Import the created-at timestamp

[mzur]

Extend this module to also import the created-at timestamp of an annotation label. The timestamp should also be used for the annotation inserted into the DB. If the annotation in the iFDO has multiple labels, take the lowest timestamp for the annotation.

This also required an update to biigle/core as the metadata Annotation and LabelAndUser objects do not support timestamps yet. Currently the timestamps are set to the time of the import.