u1db.open() and db.sync() should take URLs

At the moment, u1db.open assumes the argument is just a local path, and Database.sync() assumes the path is a remote HTTP URL.

If we made those more flexible, it would make the code for u1db-client clearer. u1db-client right now has to inspect the paths to see if they start with 'http://' and then do the right thing. That feels like something that should be abstracted away.

Note that we may not have much coverage of Database.sync() in test_backend.py, since c_backend_wrapper.pyx doesn't even have that attribute.