Dlhub-gateway

* Inherit the server `PATH` when launching a new kernel via POST request
with custom environment variables
* Fix kernel cleanup upon SIGTERM

* Add support for auth token as a query parameter

* Add command line option to whitelist environment variables for `POST /api/kernels`
* Add support for HTTPS key and certificate files
* Improve the flow and explanations in the `api_intro` notebook
* Fix incorrect use of `metadata.kernelspec.name` as a language name instead of
`metadata.language.info`
* Fix lingering kernel regression after Ctrl-C interrupt
* Switch to a conda-based dev setup from docker

* Fix compatibility with Notebook 4.3 session handler `create_session` call

* Add LICENSE file to package distributions

* Add an option to force a specific kernel spec for all requests and seed notebooks
* Add support for specifying notebook-http APIs using full Swagger specs
* Add option to serve static web assets from Tornado in notebook-http mode
* Add command line aliases for common options (e.g., `--ip`)
* Fix Tornado 4.4 compatbility: sending an empty body string with a 204 response