Alpine Spruce (Picea alpestris) (beta)

This is a refactoring of Spruce to work with modern python-jss and to negate the need for separate installations of python-jss for JSSImporter and Spruce. Please test!

• Now requires python-jss 2.0.0 or above,
  as installed via JSSImporter 1.0.0 or above,
  i.e. in the folder /Library/Application Support/JSSImporter.
  python-jss installed from pip or from source will be ignored.
• The default preferences are now taken directly from the AutoPkg preferences,
  i.e. com.github.autopkg.
• Various sections of code had to be refactored to function with the upstream
  changes to python-jss (and for @grahampugh to understand it...)