add isTransparentRedirect function

Further to discussion on google groups it seems that the first part of better support for transparent redirects is to have a simple way to check if the gateway is of this sort. 

You mentioned adding the property $isTransparentRedirect - but the calling function would need to test whether that property exists - which seems less elegant than the pattern you have used elsewhere - so I propose adding a dummy function to the parent Gateway that looks like the isRedirect function