Updated wp_asset_manager_upload_exec module

This PR updates the wp_asset_manager_upload_exec module

You can get the vulnerable version only via SVN.
Execute the following commands in your `wp-content/plugins/` folder:
```
svn co http://plugins.svn.wordpress.org/asset-manager/tags/0.2/ asset-manager
chown -R www-data:www-data asset-manager/
```

After this, activate the plugin in the backend (Plugin activation did not work correctly on my side, but enough to make the exploit work)

Output:
![11](https://cloud.githubusercontent.com/assets/105281/3740237/1e3c2904-1753-11e4-9c58-6a9e36a3d4f2.png)