Presumably my final changes for now

With these changes the lib does everything I need in productive use, so these suggested changes presumably willl be my last ones for now. 

I removed an accidental python 3 function annotation for the sake of Python 2 compatibility and added some small features (saving cards, adding URL attachments to cards) and extended the WebHook API to allow passing the API key (default behaviour not changed) when creating webhooks.
the "and getattr(self, field)" is actually probably wrong, because it's also valid to set fields to false-evaluating values like "" (e.g. empty description). So if you decide to accept these changes, please make sure to remove the second part of the "and" clause.