Merge pull request #412 from strk/libravatar-source

Add support for using "libravatar" as the GravatarSource

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
 	}