Pygnmi

- Added possibility to extract certificate from the destination network function.

- Added documentation in module regading supported the different paths naming conventions. Supported options: ``yang-module:container/container[key=value]``, ``/yang-module:container/container[key=value]``, ``/yang-module:/container/container[key=value]``, ``/container/container[key=value]``

- Fixed `gNMI Path issue <https://github.com/akarneliuk/pygnmi/issues/13>`_.

- Replaced the ``sys.exit`` with raising exceptions.
- Minor bug fix.
- Brought the gNMI path to the canonical format: ``/origin:element1/element2...``.
- Added possibility to omit the YANG module name, as some vendors doesn't include that in the request per their gNMI implementation: ``/element1/element2...``.

- Minor bug fix.

- Minor bug fix.