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....
If you pass in 'withCredentials=true' then it would need allow-credentials and it looks like PouchDB is for some reason choosing to do that by default. Which is actually a security hole. But it would be outrageously trivial for us to fix this. When we write our adapter (which we have to do) we have access to the native XHR object that pouch will use (no shimming) and can manually set withCredentials=false.