Yamcs-cli

* dbshell: Add new commands: `delimiter`, `rehash`, `source`, `status`
* dbshell: Print NULL instead of None

* Use storage URLs of the form `ys://BUCKET/OBJECT` instead of `BUCKET://OBJECT`
* Fix issue when the provided Server URL uses a trailing slash
* Fix exception during logout

* Usability fix-up relating to the login subcommand.
* Improved error reporting when no server is configured.

* Login now requires a URL instead of HOST:PORT (for example https://localhost:8090). This change was done to make it more straightforward to connect to servers behind HTTPS and/or mounted on a context path.
* Added a global ``--debug`` flag, which will enable the printout of stacktraces.
* Added a subcommand extension mechanism.
* Make use of implicit namespace packages.

Note: if you do not use pip, but instead install yamcs-cli from a clone of this repository (``python setup.py develop`` or ``python setup.py install``), remove manually any ``yamcs*-nspkg.pth`` files from your site-packages (``python setup.py uninstall yamcs-cli`` will not remove such files).

* Fixes login subcommand

* Use latest client library