Feature Request #612590 - permit initialization using an existing JSGlobalContext

Feature Request #612590 - permit initialization using an existing JSGlobalContext

Usage example:

SeedEngine *eng;
void window_object_cleared (WebKitWebView  *web_view,
                            WebKitWebFrame *frame,
                            gpointer        context,
                            gpointer        arg3,
                            gpointer        user_data) {
	eng = seed_init_with_context(NULL, NULL, context);
}
....
  g_signal_connect(G_OBJECT(web_view), "window-object-cleared",
      				G_CALLBACK(window_object_cleared), NULL);