Fix async switches (#110)

If any switches are async, the subsequent code will execute before the switches are finished. This PR moves all that code to a new function, and debounces the calls to onSwitch() so it only executes
once, after all the switches finish.

Fizes #72.

tests/lib/switch-selectors.js

@@ -9,7 +9,8 @@ tape("test switchesSelectors", function(t) {
   var pjax = {
     onSwitch: function() {
       console.log("Switched")
-    }
+    },
+    state: {}
   }
 
   var tmpEl = document.implementation.createHTMLDocument()