Merge pull request #558 from adamjstewart/features/pgi

lib/spack/docs/packaging_guide.rst

lib/spack/spack/hooks/licensing.py

lib/spack/spack/package.py

var/spack/repos/builtin/packages/allinea-forge/package.py

var/spack/repos/builtin/packages/allinea-reports/package.py

var/spack/repos/builtin/packages/nag/package.py

var/spack/repos/builtin/packages/pgi/package.py