Do not show warnings @ overlay unless explicitly set (#881)

This change tweaks the feature behavior (implemented @ #790),
so it matches the intended behavior (described @ #789).

This PR would supersede survivejs/webpack-book#262.

client/index.js

@@ -94,7 +94,7 @@ var onSocketMsg = {
 	"overlay": function(overlay) {
 		if(typeof document !== "undefined") {
 			if(typeof(overlay) === "boolean") {
-				useWarningOverlay = overlay;
+				useWarningOverlay = false;
 				useErrorOverlay = overlay;
 			} else if(overlay) {
 				useWarningOverlay = overlay.warnings;