gh-pages target.

Before using this, you need to create the `gh-pages` branch like so:

mkdir foo
git checkout --orphan gh-pages
GIT_INDEX_FILE=$PWD/.git/index.gh-pages git --work-tree foo status
touch foo/.nojekyll
echo lib.protobuf-c.io > foo/CNAME
GIT_INDEX_FILE=$PWD/.git/index.gh-pages git \
  --work-tree foo add .nojekyll CNAME
GIT_INDEX_FILE=$PWD/.git/index.gh-pages git \
  --work-tree foo commit -m 'Initial gh-pages commit.'
git checkout master
rm -rf foo

Note: This target does not work on OS X. @edmonds, you're the main person who will run this. If you run OS X I'll need to tweak it a little (xargs doesn't like -r on OS X but I need to stop it from running rm w/o args).