Add frequency option in query

The frequency option can be set to daily, weekly (the default) or
monthly. It affects the amount of data that will be collected from GA on
the collector run.
- This doesn't zero out the microsecond part.
- Shadowing the module name `datatime` could be a bad idea.
- Purely a style thing, but we're using replace above, would it be clearer here? `return datetime.replace(minute=0, second=0, microsecond=0)`