Improve ssh.AirOS backend

[nemesifier]

Let's make the ssh.AirOS backend ready for production use.

• [x] use cached property package for _ubntbox and _systemcfg methods
• [x] use iwconfig parser (#36) to understand which interfaces have wireless attributes
• [x] add device uptime
• [ ] show RAM in bytes (now is kb)
• [ ] implement JSON described in #2
• [ ] implement OrderedDict
• [ ] define supported AirOS version