Metric` class changes:
* `oldest_date_datetime` changed input type, it can either be a `datetime` object or a `timedelta` object.
`PrometheusConnect` class changes:
* `start_time` and `end_time` parameters don't accept strings any more, they can only be `datetime` objects.
* similarly `chunk_size` parameter can only be a `timedelta` object
`utils.py` added
* These are some utility functions that are not needed for the core functionality of the library, but come in handy when parsing strings for datetimes or timedeltas.