Download multi pdf bills in zip

Add option for download multi pdf in zip.

![bwj7](https://cloud.githubusercontent.com/assets/3273204/4354805/32e768b8-4242-11e4-8cc8-a5db11a9d981.jpg)
![tijc](https://cloud.githubusercontent.com/assets/3273204/4354806/371bd266-4242-11e4-8a83-52619dc4a90c.jpg)
![fx0z](https://cloud.githubusercontent.com/assets/3273204/4354808/3923d23e-4242-11e4-9e6b-a5f0c4042294.jpg)
![rwoe](https://cloud.githubusercontent.com/assets/3273204/4354809/3b33fc02-4242-11e4-8e66-45eb748fa9bc.jpg)
![l8w7](https://cloud.githubusercontent.com/assets/3273204/4354811/3c895138-4242-11e4-97d2-ee3a57358550.jpg)

This relies on the zip extension that may not be available.
You'd want to wrap it into a
```php
if (class_exists('ZipArchive')) {
[…]
}
```
and either disable the functionality entirely and/or issue a message to the user.