XML Representer is back

Now only serialises XML objects (DOM, SimpleXML, etc.)

Also removed a tonne of unneccesary */* stuff from other core representers.
removed broken wilcard handling

Now a BasicRepresenter that says it can serve "*/*" will ONLY trigger if the UA says it can handle "*/*"
removed more extraneous */* handling
better catch-all handling in representers

now Representer#preference_for_... can return either the qvalue (as usual), or an array of (qvalue,actual_item)

e.g. if a client says it can accept "*/*", and representer may send back array(1.000,"text/html") to inform the framework that the "*/*" is being represented as "text/html"