add datetime-option; use OrderedDict for times

Seems to be a faint slowlier. Maybe because the OrderedDict?
bugfix: time_in_period + check_dates; skip OrderedDict
check_times: not looping over times.items()

seems to be a bit faster to just loop over the dict like:
for t in self.times:...

datetimes-option is datetime-option now.
Add in in help-string.
Add some tests for --datetime.
v0.3.0-beta