Unable to access SenderIds model

[martingrega]

I'm unable to access SenderIds model because in TextmagicRestClient.php you are converting classname to lowercase, but classnames of models are kebabcased.

    public function __get($name) {
        $name = strtolower($name); // causing problems with kebabcase named classes
        if (!isset($this->$name)) {
            $className = __NAMESPACE__ . '\\Models\\' . ucfirst($name);
            $this->$name = new $className($this);
        }
        return $this->$name;
    }

I propose to remove strtolower() call, I think it shouldn't affect other behavior.