Add AJAX headers option

[melvynhills]

If an AJAX request is sent without proper CSRF token, Rails destroys current session.
This pull request adds an ajax_headers option to the ESHQ constructor.
This option must be a key:value object. Value can also be a function.

var eshq = new ESHQ("my-channel", {
  auth_url: "/live/connect",
  ajax_headers: {
    "X-CSRF-Token": function() {
      return $("meta[name='csrf-token']").attr("content");
    }
  }
});

[bittersweet]

b578c3c fixed this for me.

var token = $('meta[name="csrf-token"]').attr('content');
es_options = {
  auth_headers: {'X-CSRF-Token': token}
}
app.es = new ESHQ('channel', es_options);