Support Airtel USSD cleanup requests

The URLs received are `GET`s with the following parameters:

```
http://the-host/api/v1/airtel/ussd?msisdn=254XXXXXXXXX&clean=cleann&error=523&SessionID=13697502734175597&MSC=254XXXXXXXXX&=&=en&=9031510005344&=&=&=postpaid&=20130528171235405&=200220130528171113956582
```