Refactor API code to use annotator-store flaskless

Everything seems to work: reading, creating, updating, deleting, searching, permissions seem to work but need better testing, and in general more tests and thorough code review would be good.

To keep a consistent style and make refactoring easier, I decided to use traversal with a `@view_defaults` class everywhere, using the defaults class to specify the `context` predicate, so as to group the views within that. For the same reasons I do not use view names but use a resource for each location (I explictly state `name=''` for clarity, though that could be removed). I think this style works out nicely.

I put some questions in the comments, just search for the question marks.

The dependency on the next (flaskless) annotator-store version has yet to be added.
I didn't bother as these class names are not actually used anywhere else, so they feel more like comments to me, but I can change them if it hurts. ;)
I do by the way think using underscores and CamelCase combined can be useful sometimes, to separate the main name from extra details. For example in resources.py I created two names AnnotationFactory_Display and AnnotationFactory_API to distinguish two uses of the same class. (A bit of an ugly hack by the way, please leave comments down there if you have suggestions for a better approach)