allow for per instance authentication details.

[airtonix]

Currently django-gcal assumes that there should only be one set of authentication details for the entire django project.

project owns all observed model instances

In my case, each user on the project will want their related observed model instances sync'd with their own google calendar account, which requires that the observer dynamically obtain the client authentication details.

user owns their subset of observed model instances

I suspect that there needs to be a change in how client details are obtained around :
https://github.com/incuna/django-gcal/blob/master/djangogcal/observer.py#L67-L74

There will also need to be a similar approach to the feed url.