Relay prototype

Considering getting this simple relay implementation into ThaliProject while I finish up the other aspects of it - to simplify folks using the HTML5 seed.   Perhaps into a new branch on ThaliProject - but then you wouldn't be able to see the diff for review.

Only one source file here for the relay - RelayWebServer.java (and RelayWebServerTest.java).  The other bits are all NanoHTTPD.  Open questions on code file headers for those....
I would argue that Access-Control-Allow-Credentials is a bug. IN other words, we should strike its support all together. The reason being that no one talking to the relay should EVER be using cookies or HTTP auth headers. Auth is via our shared secret which we can set with some random header (e.g. not authorization) and cookies are just plain evil.

What do you think?