Allow users to log into multiple projects on same provider

Fixes #100 (closed).

Note to reviewer(s): the diff is fairly large because I renamed our 'fundamental' data structure (which is used in a zillion places) from Provider to Project. If you look at the commits separately you'll see that a86e6f is mostly just a search-and-replace operation. 5c1909 contains most of the refactoring work (e.g. removal of Project.name (used to be Provider.name) and introduction of ProjectIdentifier as a replacement 'handle' for a project).