This change adds a new XhrTransport for the SockJs client implementation. This transport is based on `UndertowClient`, Undertow's HTTP client. Note that this transport can be customized with an OptionMap that is used by the Xnio worker backing the I/O communications (see http://xnio.jboss.org). Implementation tested with undertow 1.0.36, 1.1.0.RC3, 1.2.0.Beta1. Issue: SPR-12008