One of the problems is that since OAuth2 spec is still in flux,
not all OAuth2 providers did the same thing. Every provider
has their own specification. We can't really write one ClientOauth2
for all.... :( For example, Facebook did very different things.

I am currently thus putting all this kind of things directly into
the particular client. In this case, it would be Instagram. This is
probably ok as OAuth2 is much simpler than OAuth1, thus we're
not really duplicating a lot of works...

I am installing Instagram on my phone and signing up an account.
Will merge and push to rest-more shortly after everything looks good.