Add a new method for downloading attachments

[augusto-herrmann]

The Trello API has changed the way they control access to attachments, e.g. a card image. Previously, you could just do an unauthenticated GET to the attachment URL and be done with it. Now, you have to get a token that is valid for 1 hour.

This Python package should implement some functionality to help download these attachments more easily. Like maybe attachment.get_authenticated_download_url() using the credentials already present in the TrelloClient object.

Please see this announcement and this thread.