fix parsing of dateformat for not matching locale

[jebuss]

date = datetime.strptime( ret.headers['Last-Modified'], '%a, %d %b %Y %H:%M:%S GMT' ) in `starry_night.skycam.py' fails if the tame format of the local computer and that one where you request via http disagree

[maxnoe]

Maybe using dateutil.parser.parse helps, as this can guess the format. Or setting a fixed locale for the script using the locale module