Replace and restore datetime.datetime

The regular datetime.datetime implementation uses the gettimeofday(2)
syscall (when available) for `now` and `utcnow`.  This overrides the
`now` and `utcnow` methods to call `time.time` instead (like
`datetime.today`).

Additionally, unit tests were added for `datetime.today`,
`datetime.now`, and `datetime.utcnow`.