proposed changes for outgoing messages

In some countries. It is expensive sending messages from one place to another, in here we propose that we add an extra setting for your `settings.py` where, suppose an organization received messages that cost up to 15 cents by GSM modem, they can easily send out messages as low as 1 cent through clickatell.

We propose that one can add some settings to their `localsettings.py` or `settings.py` files.

```python
OUTGOING_BACKEND_NAME = 'kannel-usb0-smsc'
```

This `OUTGOING_BACKEND_NAME` should match one of the `INSTALLED_BACKENDS` keys; i.e.

```python
OUTGOING_BACKEND_NAME = 'kannel-usb0-smsc'

INSTALLED_BACKENDS = {
    "message_tester": {
        "ENGINE": "rapidsms.backends.database.DatabaseBackend",
    },
    #  "kannel-fake-smsc": {
    #     "ENGINE":  "rapidsms.backends.kannel.KannelBackend",
    #     "sendsms_url": "http://127.0.0.1:13013/cgi-bin/sendsms",
    #     "sendsms_params": {"smsc": "FAKE",
    #                        "from": "123",  # not set automatically by SMSC
    #                        "username": "rapidsms",
    #                        "password": "rapidsms"},  # or set in localsettings.py
    #     "coding": 0,
    #     "charset": "ascii",
    #     "encode_errors": "ignore",  # strip out unknown (unicode) characters
    # },
    "kannel-usb0-smsc": {
        "ENGINE":  "rapidsms.backends.kannel.KannelBackend",
        "sendsms_url": "http://127.0.0.1:13013/cgi-bin/sendsms",
        "sendsms_params": {"smsc": "usb0-modem",
                           "from": "263771446611",  # not set automatically by SMSC
                           "username": "rapidsms",
                           "password": "rapidsms"},  # or set in localsettings.py
        "coding": 0,
        "charset": "ascii",
        "encode_errors": "ignore",  # strip out unknown (unicode) characters
       "delivery_report_url": "http://127.0.0.1:8000",
    },
}
```