day_263__diving_back_in.mdwn

After an intense week away, I didn't mean to work on git-annex today, but I
got sucked back in..

Worked on some plumbing commands for mass repository creation.
Made `fromkey` be able to read a stream of files to create from stdin.
Added a new `registerurl` plumbing command, that reads a stream of keys and
urls from stdin.