Merge Greg Becker's build database support into develop.

lib/spack/llnl/util/filesystem.py

lib/spack/llnl/util/lock.py

lib/spack/spack/__init__.py

lib/spack/spack/cmd/__init__.py

lib/spack/spack/cmd/deactivate.py

lib/spack/spack/cmd/diy.py

lib/spack/spack/cmd/extensions.py

lib/spack/spack/cmd/find.py

lib/spack/spack/cmd/fsck.py

lib/spack/spack/cmd/install.py

lib/spack/spack/cmd/module.py

lib/spack/spack/cmd/uninstall.py

lib/spack/spack/database.py

lib/spack/spack/directory_layout.py

lib/spack/spack/error.py

lib/spack/spack/package.py

lib/spack/spack/packages.py

lib/spack/spack/spec.py

lib/spack/spack/test/__init__.py

lib/spack/spack/test/database.py