import has old datetime_parser reference

[christhompson]

At https://github.com/christhompson/loredb/blob/master/loredb.py#L197, there's an untested use of a non-existent (anymore) datetime_parser.

This should be added back (or replaced), and some tests covering this functionality added to prevent the regression (since this is a very rarely used function).