iso8601: FixedOffset objects can not be unpickled

[wschaub]

This patch replaces the init of FixedOffset with a set method instead. This is so we can unpickle datetime objects using this tzinfo based class as a tzinfo= argument. you can read all about it at https://docs.python.org/2/library/datetime.html#tzinfo-objects

I ran into this while trying to unpickle a pickled dictionary filled with UrlSetElement objects that I had read in from the sitemap library which had a lastmod set in the parsed sitemap you get a nasty traceback complaining about init requiring 4 arguments but only one given with out much useful info on what inside the pickle file caused it.