And revert context-related changes to importer.go so the other
importers work again.
The problem is picago calls take an *http.Client, so we need to provide
one which already has a transport set up correctly wrt to authentication.
In particular, it needs to have the access token that was obtained
during oauth.
Change-Id: I7b83cd80910111a9abf1c192cc59841f6d818fb7
Reorganizes common OAuth2 things into importer/oa2_importers.go
Rewrites CallbackURLParameters to return url.Values.
A controversial workaround for nil context has been left out,
and this may cause panic and will need a proper fix.
Change-Id: I0789f4732412aa559078ec2c6b6797925ab8cd05
Works but only lightly tested.
Could use lots of refactoring with code common with twitter.
Needs a test too.
Change-Id: I07f72d29acbc0226aeeedba0afe0bf7eeeac5ad3
mpl