Make DropboxRepository configurable using URL

Libearth recently registered an [entry point][1] group named `libearth.repositories` to make repositories possible to be configured using URLs like `file:///home/dahlia/.earthreader/`.  See also the [related docs][2].

This patch implements the protocol for `DropboxRepository`.  So it can be configured using URL like:

    dropbox://access-token@/folder/path/

Note that it also adds `ez_setup.py` that has been missing while `setup.py` tries to load (c63148b6dee23b8e05a195b8283c7238bd4a637c).

[1]: https://pythonhosted.org/setuptools/pkg_resources.html#entry-points
[2]: http://libearth.earthreader.org/en/latest/libearth/repository.html