urls resolved via urlresolver are not yet guaranteed be ok and playable.

this can lead to autoplay failing in case stream_url in PlaySource()
is resolved, but for other reasons the url object itself fails to play
(is not reachable).

new function url_is_playable(URL) sends HTTP HEAD request to target
server/url to physically check url's validity and availablity before
it is passed later to XBMC for play.

(a good test example for this is TV SHOW Scrubs, S06E06)