loadUrl enhancements

- Make `options` parameter optional
- Allow partial overriding of instance options when calling `loadUrl` directly
- Make `requestOptions` optional
- Document `loadUrl` usage and provide examples

lib/proto/attach-form.js

@@ -1,5 +1,5 @@
 var on = require("../events/on")
-var clone = require("../clone")
+var clone = require("../util/clone")
 
 var attrClick = "data-pjax-click-state"