Improve context package

You shouldn't have to import both:

  github.com/docker/distribution/context
  golang.org/x/net/context

just to use the distribution tools and implement the distribution interfaces.

By pulling the Context interface from golang.org/x/net/context into the
context package within the distribution project, you no longer have to import
both packages.

Note: You do not have to change anything anywhere else yet! All current uses
of both packages together will still work correctly because the Context
interface from either package is identical.

I've also made some other minor changes:

- Added a RemoteIP function. It's like RemoteAddr but discards the port suffix
- Added `.String()` to the response duration context value so that JSON log
  formatting shows human-parseable duration and not just number of nano-seconds
- Added WithMapContext(...) to the context package. This is a useful function
  so I pulled it out of the main.go in cmd/registry so that it can be used
  elsewhere.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

configuration/configuration_test.go

@@ -17,11 +17,11 @@ func Test(t *testing.T) { TestingT(t) }
 var configStruct = Configuration{
 	Version: "0.1",
 	Log: struct {
-		Level     Loglevel          `yaml:"level"`
-		Formatter string            `yaml:"formatter"`
-		Fields    map[string]string `yaml:"fields"`
+		Level     Loglevel               `yaml:"level"`
+		Formatter string                 `yaml:"formatter"`
+		Fields    map[string]interface{} `yaml:"fields"`
 	}{
-		Fields: map[string]string{"environment": "test"},
+		Fields: map[string]interface{}{"environment": "test"},
 	},
 	Loglevel: "info",
 	Storage: Storage{
@@ -340,7 +340,7 @@ func copyConfig(config Configuration) *Configuration {
 	configCopy.Version = MajorMinorVersion(config.Version.Major(), config.Version.Minor())
 	configCopy.Loglevel = config.Loglevel
 	configCopy.Log = config.Log
-	configCopy.Log.Fields = make(map[string]string, len(config.Log.Fields))
+	configCopy.Log.Fields = make(map[string]interface{}, len(config.Log.Fields))
 	for k, v := range config.Log.Fields {
 		configCopy.Log.Fields[k] = v
 	}