Made some api changes.
listImportableContents needs to provide the size
of the data, so the downloader can check disk free space.
retrieveExportWithContentIdentifier is passed the filepath to write to
Use temporary "CID" key during download of a ContentIdentifier from a
remote, so withTmp can be used and then move the content to the real key
once it's known.
Added graftTree but it's buggy.
Should use graftTree in Annex.Branch.graftTreeish; it will be faster
than the current implementation there.
Started Annex.Import, but untested and it doesn't yet handle tree
grafting.
