Parse time formats better #4

Some gpx files use the time format <time>2010-07-18T15:21:41+01:00</time> which 
breaks your script at the moment.

You might consider using the parser from the python dateutil module which will 
handle virtually any time format. 

e.g.:

from dateutil import parser

def prettify_time(time):
    time = parser.parse(time)
    if tzname:
        time=time.replace(tzinfo=pytz.utc)
        time=time.astimezone(pytz.timezone(tzname))
    return time