Pushing an Image causes the docker client to give an error message instead of
writing out streamed status.

This is caused by a Buffering message that is not in the correct json format:

[...]
{"status"
:"Pushing 6bba11a28f1ca247de9a47071355ce5923a45b8fea3182389f992f4
24b93edae"}Buffering to disk 244/? (n/a)..
{"status":"Pushing",[...]

The "Buffering to disk" message is originated in
srv.runtime.graph.TempLayerArchive

I am now using the StreamFormatter provided by the context from which the
method is called.
Merge pull request #1066 from mhennings/fix-broken-streaming-result

* Server: Fix streaming status to the docker client while pushing images