implement interstitial source (issue #19)
    
      * allows bypassing of screencap and/or camera
      * factored gStreamer functinoality out of AvCaster class and into new Gstreamer class
      * renamed AvCaster::InitGstreamer() to Gstremer::Initialize()
      * renamed AvCaster::SetGstreamerState() to Gstremer::SetState()
      * added Gstreamer::InterstitialBin and Gstreamer::ConfigureInterstitial() (snow only)
      * added Gstreamer::AddBin() Gstreamer::RemoveBin() Gstreamer::IsInPipeline() helpers