Pull bagit into its own repo

[jechols]

bagit code could easily live independently of the rest of this monster-repo, and it would probably be useful to extract it, given it's now pretty much got a turnkey binary as well as being a usable Go package. Being trapped in this repo also makes it annoying for the same reasons we've had with other stuff in here: if an app uses two of the packages here, but needs one of them pinned to a certain version, ... well that's just not an option.