Check carrier name conditionally

Carrier name is only available on iOS7+
Add more carrier info to the log

Since some carriers will appear as 'Carrier', add the country code, MCC,
and MNC.

Result looks like:

```
Reachability - WordPress.com - WiFi: Y  WWAN: N  Carrier: Carrier [ES/214/19]  Type: CTRadioAccessTechnologyHSDPA
```

refs #638
Merge pull request #670 from wordpress-mobile/issue/638-carrier-ios6

#638 - Check carrier name conditionally