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....
Yeah, wildcard is invalid if allow-credentials is true, although a few browsers don't seem to care so I default back to wildcard in case it buys me any ground with a malformed request (cors with no origin header), but in practice it is probably a toss up between doing this (which lets your XHR explain the CORS error) or just returning an outright error code.