Minor formatting updates to README
Add documentation for multiple keys
migrate-key: take separate arguments for old key and new key

I don't want to encourage people to overwrite their old keys
until they've successfully unlocked their repository with the
migrated key.
Update 'git-crypt help' message

Documented new arguments to 'git-crypt migrate-key' and adjusted spacing.
Fix formatting in NEWS.md
Add .gitattributes file to ignore .git files when creating archive
Update README, NEWS, write release notes for 0.4