SPACK-59: Documentation updates, bugfix in fetching.

lib/spack/docs/.gitignore

lib/spack/docs/Makefile

lib/spack/docs/basic_usage.rst

lib/spack/docs/command_index.in

lib/spack/docs/features.rst

lib/spack/docs/index.rst

lib/spack/docs/mirrors.rst

lib/spack/docs/packaging_guide.rst

lib/spack/docs/site_configuration.rst

lib/spack/spack/mirror.py

lib/spack/spack/package.py

lib/spack/spack/test/git_fetch.py

lib/spack/spack/test/hg_fetch.py

lib/spack/spack/test/svn_fetch.py