vxJS.xhr() allows serialization of parameters into plain request
parameters; tries to mimick PHP conventions - with other server side
languages one should stick to JSON encoding; new configuration
parameter: use JsonSerialization; defaults to false, thus breaking
downwards compatibility;
minor updates to vxJS.xhrForm(); comments updated

Change-Id: Ifb9fbbbc0da2f64f8de02b3f6ffa4ffd21972030