Improve push/pull/import to json stream

Currently push/pull/import

Uses chunked http, not very handy when you want to parse it, for exemple to display a nice progress bar if you build a non-console client.

We could use json streaming, it's quite standard look like this:

    {"status" : "Pulling repository busybox from https://index.docker.io/v1"}
    {"status" : "Pulling image e9aa60c60128cad1 (latest) from busybox"}
    {"status" : "Pulling e9aa60c60128cad1 metadata"}
    {"status" : "Pulling e9aa60c60128cad1 fs layer"}
    {"progress" : "5778/? (n/a)"}
    ...


I implemented the pull command using json stream on v1.1
So /v1/ will be chunked http and /v1.1/ json stream.
The client decodes using the content-type header.

It's pretty basic for now, if the future we could add a "error" key, to know when the pull went bad.
Feel free to comment.