add git-annex addcomputed

add git-annex addcomputed

Working pretty well. Mostly. But:

* Does not yet support inputs that are non-annexed files checked into git
* --fast is currently broken (will need something like VURL keys)
* --unreproducible still uses a checksumming backend, so drop and get
  again will likely fail (needs probably to use an URL key or something
  like one)

The compute special remote seems to work pretty well too. Eg,
getting from it works, and dropping content that is present in it works.