Exempt `master` from deleted branches

This is helpful for those who use the `git-flow` branch model and might
be surprised to find that this script deletes their master branch.
Update docs to match `git-delete-merged-branches` behavior
Merge pull request #240 from egrim/bug/delete-merged-branches-clobbers-master

Exempt `master` from being deleted by `git-delete-merged-branches`.