Format times in inspect command with a template as RFC3339Nano

In 1.6.2 we were decoding inspect API response into interface{}.
time.Time fields were JSON encoded as RFC3339Nano in the response
and when decoded into interface{} they were just strings so the inspect
template treated them as just strings.
From 1.7 we are decoding into types.ContainerJSON and when the template
gets executed it now gets a time.Time and it's formatted as
2015-07-22 05:02:38.091530369 +0000 UTC.
This patch brings back the old behavior by typing time.Time fields
as string so they gets formatted as they were encoded in JSON -- RCF3339Nano

Signed-off-by: Antonio Murdaca <runcom@linux.com>

api/types/types.go

@@ -86,7 +86,7 @@ type ImageInspect struct {
 	Id              string
 	Parent          string
 	Comment         string
-	Created         time.Time
+	Created         string
 	Container       string
 	ContainerConfig *runconfig.Config
 	DockerVersion   string
@@ -215,14 +215,14 @@ type ContainerState struct {
 	Pid        int
 	ExitCode   int
 	Error      string
-	StartedAt  time.Time
-	FinishedAt time.Time
+	StartedAt  string
+	FinishedAt string
 }
 
 // GET "/containers/{name:.*}/json"
 type ContainerJSONBase struct {
 	Id              string
-	Created         time.Time
+	Created         string
 	Path            string
 	Args            []string
 	State           *ContainerState