Dlhub-gateway

* Introduce an [API for developing mode plug-ins](https://jupyter-kernel-gateway.readthedocs.io/en/latest/plug-in.html)
* Separate `jupyter-websocket` and `notebook-http` modes into plug-in packages
* Move mode specific command line options into their respective packages (see `--help-all`)
* Report times with respect to UTC in `/_api/activity` responses

* Switch HTTP status from 402 for 403 when server reaches the max kernel limit
* Explicitly shutdown kernels when the server shuts down
* Remove `KG_AUTH_TOKEN` from the environment of kernels
* Fix missing swagger document in release
* Add `--KernelGateway.port_retries` option like in Jupyter Notebook
* Fix compatibility with Notebook 4.2 session handler `create_session` call

* Backport `--KernelGateway.port_retries` option like in Jupyter Notebook
* Fix compatibility with Notebook 4.2 session handler `create_session` call

* Support multiple cells per path in `notebook-http` mode
* Add a Swagger specification of the `jupyter-websocket` API
* Add `KERNEL_GATEWAY=1` to all kernel environments
* Support environment variables in `POST /api/kernels`
* numpydoc format docstrings on everything
* Convert README to Sphinx/ReadTheDocs site
* Convert `ActivityManager` to a traitlets `LoggingConfigurable`
* Fix `base_url` handling for all paths
* Fix unbounded growth of ignored kernels in `ActivityManager`
* Fix caching of Swagger spec in `notebook-http` mode
* Fix failure to install due to whitespace in `setup.py` version numbers
* Fix call to kernel manager base class when starting a kernel
* Fix test fixture hangs

* Backport `--KernelGateway.port_retries` option like in Jupyter Notebook
* Fix compatibility with Notebook 4.2 session handler `create_session` call

* Enable `/_api/activity` resource with stats about kernels in `jupyter-websocket` mode
* Enable `/api/sessions` resource with in-memory name-to-kernel mapping for non-notebook clients that want to look-up kernels by associated session name
* Fix prespawn kernel logic regression for `jupyter-websocket` mode
* Fix all handlers so that they return application/json responses on error
* Fix missing output from cells that emit display data in `notebook-http` mode