Major changes to the build process and the images offered, while still retaining (mostly) compatibility with the
previous images.  This release shrinks the image size by up to 64%, by doing the following:

* Builds are now based off of the official Debian "slim" image (tracks the latest release)
* Image tagged 'slim-dev-base' is a very basic Jupyter-enabled image. Probably not so useful for real use, but
  great as an image on which to build others.
* The 'slim-dev' image is a nearly complete replacement for the old hunter image's Python 3 environment.  It has NO
  Python 2 support, nor any Spark support.  If you don't care about Spark, this is the version you should use, as it
  is the smallest useful hunting image.
* The 'slim-dev-spark' image is built upon 'slim-dev', but adds Apache Spark and Mesos support nearly identical to
  that offered by the official Jupyter 'pyspark-notebook' image, but with a much smaller image.

In addition the splitting the build into three images and reducing the sizes, the following changes were made:

* Moved to a multi-stage build process, to speed things up when we don't need to rebuild the base image.
* Modified the Makefile to support the new build changes
* Added a separate test notebook for testing Spark

* Now use debian:stretch-slim as the base image, because otherwise we have to deal with updating
  JRE version whenever Debian decides to do it (e.g. now install openjdk-11-jre-headless instead
  of openjdk-8-jre-headless).  Also, Mesos has issues on newer Debian due to libcurl3 dependencies
  that aren't fixed in the Mesos distribution yet.
* Removed unnecessary version info on node packages installed by NPM.