Change the Collection API

The new API used comes from vdirsyncer, as proposed by @untitaker in
issue #130.

The code has been tested and works with the (too simple) unit tests, and
with Lightning and DAVdroid. Many things are broken and a good part of
the code has not be ported to the new API yet. TODOs have been added
where the application is known to be broken.

tests/test_base.py

@@ -117,7 +117,6 @@ class TestCustomStorageSystem(BaseRequests, BaseTest):
         radicale.config.set("storage", "type", "tests.custom.storage")
         from tests.custom import storage
         storage.FOLDER = self.colpath
-        storage.GIT_REPOSITORY = None
         self.application = radicale.Application()
 
     def teardown(self):