Adds function for retrieving mac address

This PR adds the following API call to `wifi-cc3000`:

```.js
// Returns a 6 byte buffer with the mac address or null if there was an error
wifi.macAddress()
```

It may be better to accept a callback for (`function(err, macAddress)`). I thought no callback would be simpler but I'm open to other thoughts.
Just `colony_createbuffer` in C instead of creating one in JS and passing it in.