add ability to fetch pkpass from the URL

it seems some websites detect the browser user-agent, so doing something like:

```
curl -L -O -H "User-Agent: Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_2 like Mac OS X; nl-nl) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8H7 Safari/6533.18.5" $1
```

helps. Of course, it's not very enjoyable to do it on the phone :)

Would be thou nice have option as **Import URL**