Feature/handle ussd session

- Handles ussd session parameter
- Uses callback parameter to determine session_event
Rather write this like so:
```python
    values[field] = request.args.get(field)[0].decode(
        self.ENCODING)
```
Alternatively, put `request.args.get(field)[0]` in a local variable or something and use that on the next line to avoid going over 79 characters.