Allow adding MemoryImage using URL

The current version of PHPWord only allow dynamic image when the source string contains `.php`. When `allow_url_open = on`, the GD library functions that we use, e.g. `imagecreatefromgif`, can get [images from URL](http://www.php.net/manual/en/function.imagecreatefromgif.php).
I've also done the following things:
- Create 2 more samples from the `old` folder
- Create unit test for PHPWord\Settings #113 :)