Stop the webchat events firing on every ajax check

previous functionality was to send analytics on every state change,
this now only sends state changes to analytics as opposed to every
time we poll.

it alos includes lolex so we can do fake timers to test this,
recommendation from @NickColley

app/assets/javascripts/webchat/library.js

@@ -22,6 +22,7 @@
     var webchatStateClass   = 'js-webchat-advisers-'
     var responseNormaliser  = options.responseNormaliser
     var intervalID          = null
+    var lastRecordedState   = null
 
     function init () {
       if (!availabilityUrl || !openUrl) throw 'urls for webchat not defined'
@@ -33,7 +34,7 @@
     function handleOpenChat (evt) {
       evt.preventDefault()
       global.open(openUrl, 'newwin', 'width=200,height=100')
-      GOVUK.analytics.trackEvent('webchat', 'opened')
+      trackEvent('opened')
     }
 
     function checkAvailability () {
@@ -65,7 +66,15 @@
       var currentState = $el.find("." + webchatStateClass + state)
       $el.find('[class^="' + webchatStateClass + '"]').addClass('hidden')
       currentState.removeClass('hidden')
+      trackEvent(state)
+    }
+
+    function trackEvent (state) {
+      state = state.toLowerCase()
+      if (lastRecordedState === state) return
+
       GOVUK.analytics.trackEvent('webchat', state)
+      lastRecordedState = state
     }
 
     init()