Embed NB2KG into Notebook server

This change alleviates a significant pain-point for consumers of Jupyter
Kernel and Enterprise Gateway projects by embedding the few classes defined
in the NB2KG server extension directly into the Notebook server.  All code
resides in a separate gateway directory and the 'extension' is enabled
via a new configuration option `--gateway-url`.

Renamed classes from those used in standard NB2KG code so that Notebook
servers using the existing NB2KG extension will still work.

Added test_gateway.py to exercise overridden methods.  It does this by
mocking the call that issues requests to the gateway server.

Updated the _Running a notebook server_ topic to include a description
of this feature.

docs/source/public_server.rst

@@ -343,6 +343,35 @@ single-tab mode:
     });
 
 
+Using a gateway server for kernel management
+--------------------------------------------
+
+You are now able to redirect the management of your kernels to a Gateway Server
+(i.e., `Jupyter Kernel Gateway <https://jupyter-kernel-gateway.readthedocs.io/en/latest/>`_ or
+`Jupyter Enterprise Gateway <https://jupyter-enterprise-gateway.readthedocs.io/en/latest/>`_)
+simply by specifying a Gateway url via the following command-line option:
+
+    .. code-block:: bash
+
+        $ jupyter notebook --gateway-url=http://my-gateway-server:8888
+
+the environment:
+
+    .. code-block:: bash
+
+        GATEWAY_URL=http://my-gateway-server:8888
+
+or in :file:`jupyter_notebook_config.py`:
+
+   .. code-block:: python
+
+      c.NotebookApp.gateway_url = http://my-gateway-server:8888
+
+When provided, all kernel specifications will be retrieved from the specified Gateway server and all
+kernels will be managed by that server.  This option enables the ability to target kernel processes
+against managed clusters while allowing for the notebook's management to remain local to the Notebook
+server.
+
 Known issues
 ------------