Patch: check for valid URL scheme on banners

I'm submitting a patch for the file openx_manager.banner.inc which adds validation for proper URL scheme on the banner submission form. E.g. if the form complains if the user omits http://, https://, ftp://, etc. in the banner destination URL field.

My OpenX server does not verify destinations URLs for banners, so any invalid URLs would actually get propagated to advertising channels, resulting in broken banners and invalid clicks.