* Add dns server
The access point example is a bit complicated to use as you have to open
a browser and enter a url with the IP address of the device
Add a simple dns server that can be used to make the access point behave
like a captive portal. All DNS requests point back to the gateway IP.
* Improve AP example
The web page /ledtest gives you a web page with an option to turn the
led on and off
Start the DNS server and redirect all HTTP requests to this page.
Improve the code so it can handle more than one request at a time.
Fixes: #232
