Fill missing "Channel" field

For some reason my output from 'iwlist wlan0 scanning'  is missing a channel field.

However, the channel is present at the frequency field:
       Frequency:2.457 GHz (Channel 10) 

So to use this value if it is available or the channel field if it is available this is what I did:

To solve this I changed and added these lines to scan.py:
```python
quality_re_dict = {'dBm': re.compile(r'Quality=(\d+/\d+).*Signal level=(-\d+) dBm'),
                   'relative': re.compile(r'Quality=(\d+/\d+).*Signal level=(\d+/\d+)')}
frequency_re = re.compile(r'([\d\.]+ .Hz).*Channel (\d+).*')

```
```python
normalize_value = {
     'ssid': lambda v: v.strip('"'),
#    'frequency': lambda v: frequency_re.search(v).group(1),
     'encrypted': lambda v: v == 'on',
```
```python
            if 'WPA2' in value:
                    cell.encryption_type = 'wpa2'
            elif key == 'frequency':
                setattr(cell, 'frequency'  ,frequency_re.search(value).group(1) )
                setattr(cell, 'channel'  ,int(frequency_re.search(value).group(2) ))
            elif key in normalize_value:
                setattr(cell, key, normalize_value[key](value))
```

Hope you can insert it to the code so that it might help some one else too.