JSONMessage structure definitions shouldn't pull in TTY libs

[dhiltgen]

Downstream projects that want to leverage the payloads from pull, load, etc. need some way to parse the messages coming back. This currently lives in https://github.com/moby/moby/blob/master/pkg/jsonmessage/jsonmessage.go

Unfortunately, the type definitions are intertwined with TTY code for CLI handling.

These should be split out, ideally into the client library so downstream projects can easily consume the standard structure definitions.

[sheerun]

Please :) This is also needed for parsing output of client.ImagePull

[sheerun]

Reference: docker/engine-api#89

[thaJeztah]

/cc @stevvooe @cpuguy83

[wminshew]

+1; unable to pull cleanly with dep. Going to hack around for now, but not ideal.