Bugfix 1460: preserve nanoseconds and timezones in date range from get info

[alexowens90]

THIS IS AN API CHANGE, AND SHOULD NOT BE MERGED UNTIL WE WANT TO BUMP THE MAJOR VERSION NUMBER FOR A MORE IMPORTANT FEATURE Fixes #1460 Previously the date_range returned by get_info[_batch] was a datetime.datetime, which only has microsecond precision. Now returns a pandas.Timestamp with nanosecond precision. Also keeps the timezone information which was previously discarded for unknown reasons.