Add -s (security) flag to t2 wifi

[Frijol]

Per spec:

> -s <security>: security protocol to connect with (i.e. WEP, WPA, PSK)

moved from #82

[Frijol]

Looks like some of the setup for this is already in place, just not fully implemented: in lib/tessel/commands.js:

module.exports.setNetworkEncryption = function(encryption) {
  return ['uci', 'set', 'wireless.@wifi-iface[0].encryption=' + encryption];
};

[rwaldron]

@HipsterBrown same here!