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).
Code sample:
```php
$source = 'http://php.net/images/logos/php-med-trans-light.gif';
$section->addText("Remote image from: {$source}");
$section->addMemoryImage($source);
```
I've also done the following things:
1. Create 2 more samples from the `old` folder
2. Create unit test for PHPWord\Settings #119 
3. Test & migrate the remaining old samples as per #43