Add support for using "libravatar" as the GravatarSource

Just to make it easier for administrator to configure libre avatar,
as it is done for "duoshuo" and "gravatar"

modules/setting/setting.go

 		GravatarSource = "http://gravatar.duoshuo.com/avatar/"
 	case "gravatar":
 		GravatarSource = "https://secure.gravatar.com/avatar/"
+	case "libravatar":
+		GravatarSource = "https://seccdn.libravatar.org/avatar/"
 	default:
 		GravatarSource = source
 	}