Parametrized delay allow even older Webkit browsers not to show spinners...

As noted by @eveiga in #144 and #532, in some older Android devices connecting through XHR-polling, the infinite spinner is not avoided even using the defer method to delay the opening the transport by 100ms which was added through commit 59df51f on xhr-polling.js.

This change adds an option on socket connection called ```xhr onready delay``` to allow the client to set this delay to a higher value and avoid to hardcode it on ```util.defer```. When not specified, the default 100ms will be used instead.