Support schemeless URLs for XHR and Flash files

URLs without an HTTP or HTTPS scheme, like:
  "//www.example.com/shindig/api/"
... are legal and supported in HTML.  Such URLs automatically follow the scheme of the container, and so are really handy for implementing containers that support both HTTP and HTTPS.  However, I've had reports of some issues with IE sending XHRs to schemeless URLs, and failures with flash execution when swfs are loaded from schemeless URLs.  This patch fixes up such URLs on the client.