Improve handling of time parameters

Allow for more user-friendly time formats to be given such as: 

* `metar.py KLAX 10 12` to be interpreted as "11:00" and "12:00" rather than the day, as is default with dateutil. 

* `metar.py KLAX 11` to be interpreted as "11:00 to current time" where the second time parameter is blank and "11" is not parsed as the date. 