Add SC16 tutorial to the Spack docs. (#2334)

lib/spack/docs/index.rst

lib/spack/docs/tutorial/.gitattributes

lib/spack/docs/tutorial/Spack-SC16-Tutorial.pdf

lib/spack/docs/tutorial/examples/0.package.py

lib/spack/docs/tutorial/examples/1.package.py

lib/spack/docs/tutorial/examples/2.package.py

lib/spack/docs/tutorial/examples/3.package.py

lib/spack/docs/tutorial/examples/4.package.py

lib/spack/docs/tutorial/sc16-tutorial-slide-preview.png

lib/spack/docs/tutorial_sc16.rst

lib/spack/docs/tutorial_sc16_modules.rst

lib/spack/docs/tutorial_sc16_packaging.rst

lib/spack/docs/tutorial_sc16_spack_basics.rst