loadUrl enhancements (#134)

`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-link.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"
 var attrKey = "data-pjax-keyup-state"
@@ -9,9 +9,6 @@ var linkAction = function(el, event) {
   // clone it so the changes don't persist
   var options = clone(this.options)
 
-  // Initialize requestOptions since loadUrl expects it to be an object
-  options.requestOptions = {}
-
   // Don’t break browser special behavior on links (like page in new window)
   if (event.which > 1 || event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) {
     el.setAttribute(attrClick, "modifier")