AP mode: Track client connects & disconnects

Placeholder for a pull request - I have written some code that combines the SimpleLinkCallbacks.cpp in utility/ with some new static variables and functions in WiFiClass which track client connects (IP/MAC) and disconnects.

A user may poll and inform about these using code similar to this:
```
  static int knowndevs = 0;
  int nowdevs = 0;

  nowdevs = WiFi.getTotalDevices();
  if (nowdevs > knowndevs) {
    Serial.print("Client connected!  IP = ");
    WiFi.getLatestDevice().printTo(Serial);
    Serial.println();
  }
  if (nowdevs < knowndevs) {
    Serial.println("Client disconnected.");
  }
  knowndevs = nowdevs;
```

Attaching my phone to this AP and then disconnecting, the output looks like:
```
Client connected!  IP = 192.168.1.2
Client disconnected.
```