I put it in ~/.ssh/ because there's no reliable way to get it into PATH,
and OSX ssh doesn't even honor user's PATH by default.
authorized_keys generators will need to check if it's there. Not done yet.
I'm guessing that ~/Desktop/git-annex.app will be visiable.
For the system-wide installation, I don't know where to put it, though
somewhere in /Library seems likely.