Change CreationTimestamp to a util.Time and set in each storage implementation

Because time.Time doesn't work correctly with our YAML package, it is necessary
to introduce a type, util.Time, which serializes correctly to JSON and YAML.

Eventually we would like timestamping to cut across storage implementations;
for now, we set it in each storage.
Merge pull request #856 from pmorie/workaround-timestamp

Set CreationTimestamp in each storage implementation