initial docs for find and flake8 cleanup

lib/spack/docs/basic_usage.rst

lib/spack/spack/cmd/find.py

lib/spack/spack/database.py

lib/spack/spack/spec.py

var/spack/repos/builtin/packages/py-autopep8/package.py

var/spack/repos/builtin/packages/py-pep8/package.py