install() now takes spec AND prefix

lib/spack/spack/directory_layout.py

lib/spack/spack/package.py

lib/spack/spack/packages/callpath.py

lib/spack/spack/packages/cmake.py

lib/spack/spack/packages/dyninst.py

lib/spack/spack/packages/libdwarf.py

lib/spack/spack/packages/libelf.py

lib/spack/spack/packages/libunwind.py

lib/spack/spack/packages/mpich.py

lib/spack/spack/packages/mpileaks.py

lib/spack/spack/spec.py

lib/spack/spack/test/mock_packages/callpath.py

lib/spack/spack/test/mock_packages/dyninst.py

lib/spack/spack/test/mock_packages/fake.py

lib/spack/spack/test/mock_packages/libdwarf.py

lib/spack/spack/test/mock_packages/libelf.py

lib/spack/spack/test/mock_packages/mpich.py

lib/spack/spack/test/mock_packages/mpich2.py

lib/spack/spack/test/mock_packages/mpileaks.py

lib/spack/spack/test/mock_packages/zmpi.py