Finish "git annex watch" command, which runs, in the background, watching via
inotify for changes, and automatically annexing new files, etc.

There is a `watch` branch in git that adds such a command, although currently
it only handles adding new files, and nothing else. To make this really
useful, it needs to:

- notice deleted files and stage the deletion
  (tricky; there's a race with add..)
- notice renamed files, auto-fix the symlink, and stage the new file location
- periodically auto-commit staged changes
- honor .gitignore, not adding files it excludesa

Also to do:

- Support OSes other than Linux; it only uses inotify currently.
  OSX and FreeBSD use the same mechanism, and there is a Haskell interface
  for it,