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....
So unfortunately stopServer doesn't actually work in Java . See https://github.com/couchbase/couchbase-lite-java-listener/issues/43

This is why in our other tests you will see the ThaliListener being created statically and then re-used. Which sucks from a test cleanliness perspective but until that bug gets fixed we don't have much choice. :(