Add .docker/config.json and support for HTTP Headers

This PR does the following:
- migrated ~/.dockerfg to ~/.docker/config.json. The data is migrated
  but the old file remains in case its needed
- moves the auth json in that fie into an "auth" property so we can add new
  top-level properties w/o messing with the auth stuff
- adds support for an HttpHeaders property in ~/.docker/config.json
  which adds these http headers to all msgs from the cli

In a follow-on PR I'll move the config file process out from under
"registry" since it not specific to that any more. I didn't do it here
because I wanted the diff to be smaller so people can make sure I didn't
break/miss any auth code during my edits.

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/sources/reference/commandline/cli.md

@@ -48,6 +48,35 @@ These Go environment variables are case-insensitive. See the
 [Go specification](http://golang.org/pkg/net/http/) for details on these
 variables.
 
+## Configuration Files
+
+The Docker command line stores its configuration files in a directory called
+`.docker` within your `HOME` directory. Docker manages most of the files in
+`.docker` and you should not modify them. However, you *can modify* the
+`.docker/config.json` file to control certain aspects of how the `docker`
+command behaves.
+
+Currently, you can modify the `docker` command behavior using environment 
+variables or command-line options. You can also use options within 
+`config.json` to modify some of the same behavior.  When using these 
+mechanisms, you must keep in mind the order of precedence among them. Command 
+line options override environment variables and environment variables override 
+properties you specify in a `config.json` file.
+
+The `config.json` file stores a JSON encoding of a single `HttpHeaders`
+property. The property specifies a set of headers to include in all
+messages sent from the Docker client to the daemon. Docker does not try to
+interpret or understand these header; it simply puts them into the messages.
+Docker does not allow these headers to change any headers it sets for itself.
+
+Following is a sample `config.json` file:
+
+    {
+      "HttpHeaders: {
+        "MyHeader": "MyValue"
+      }
+    }
+
 ## Help
 To list the help on any command just execute the command, followed by the `--help` option.