Add support for setting http referrer

- Add url option to specify the http referrer
- Add httpReferrer attribute to webview

NOTE: This is still not complete. Some love has to be done to
guest-view-manager.coffee and very likely the function calls called
createGuest and to the code that uses them.

atom/browser/lib/guest-view-manager.coffee

@@ -47,7 +47,7 @@ createGuest = (embedder, params) ->
     max = width: params.maxwidth, height: params.maxheight
     @setAutoSize params.autosize, min, max
     if params.src
-      @loadUrl params.src
+      @loadUrl params.src, params.urlOptions
     if params.allowtransparency?
       @setAllowTransparency params.allowtransparency