User timestamps (#28)

* Add created_at to user

* Add updated_at to user

* Add last_login to user

management/user.go

@@ -1,6 +1,9 @@
 package management
 
-import "encoding/json"
+import (
+	"encoding/json"
+	"time"
+)
 
 type User struct {
 
@@ -23,6 +26,15 @@ type User struct {
 	// for users to be added to SMS connections.
 	PhoneNumber *string `json:"phone_number,omitempty"`
 
+	// The time the user is created.
+	CreatedAt *time.Time `json:"created_at,omitempty"`
+
+	// The last time the user is updated.
+	UpdatedAt *time.Time `json:"updated_at,omitempty"`
+
+	// The last time the user has logged in.
+	LastLogin *time.Time `json:"last_login,omitempty"`
+
 	// UserMetadata holds data that the user has read/write access to (e.g.
 	// color_preference, blog_url, etc).
 	UserMetadata map[string]interface{} `json:"user_metadata,omitempty"`