Make DropboxRepository configurable using URL

Libearth recently register 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`.

[1]: https://pythonhosted.org/setuptools/pkg_resources.html#entry-points
[2]: http://libearth.earthreader.org/en/latest/libearth/repository.html