Move all code that executes after the switches to a new function

If any switches are async, the subsequent code will execute before
the switches are finished. This commit 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.

lib/switches-selectors.js

@@ -24,6 +24,9 @@ module.exports = function(switches, switchesOptions, selectors, fromEl, toEl, op
       if (this.log) {
         this.log("newEl", newEl, "oldEl", oldEl)
       }
+
+      this.state.numPendingSwitches++
+
       if (switches[selector]) {
         switches[selector].bind(this)(oldEl, newEl, options, switchesOptions[selector])
       }