Pygnmi

- Default value for ``encoding`` everywhere is set to ``None``.
- Method ``capabilities()`` now is called as part of ``connect()`` to collect supported encoding as part of session establishing.
- If ``encoding`` is not specified by user, then it is auto-set based on the list collected via ``capabilites()`` with ``json`` having the first priority follwed by ``json_ietf``.

- Added new argument ``-e / --encoding`` to ``pygnmicli`` to specify the encoding, which overrides the default one. Fix for `Issue 58 <https://github.com/akarneliuk/pygnmi/issues/58>`_.
- Fixed minor bug with encoding handling inside ``get()`` and ``subscribe2()`` methods.
- Simplified the code.

- Fixed bug, when returned ``json_val`` or ``json_ietf_val`` is not processed correctly if the value is empty string. Fix for `Issue 58 <https://github.com/akarneliuk/pygnmi/issues/58>`_.

- Fixed minor issue with establishing ``insecure`` channel.
- Fixed bug with inabillity to specify ``prefix`` in Subscribe messages for ``subscribe2()`` method.
- **Important**: It is recommended to use method ``subscribe2()`` instead of ``subscribe()`` for building telemetry collectors with ``pygnmi`` as this method is further developed and throroughle tested. The method ``subscribe()`` will be deprecated in future releases.
- Functionality ``qos`` is now properly supported in ``subscribe2()`` method.

- Fixed some issues with telemetry representation with ``pygnmicli``.

- Change logic of setting default values for some parameters to improve user experience.
- Added token authentication to pygnmicli.