Spike out an update wrapper

We can expose just a few event subscription methods on atom.update to
take care of what most packages (e.g. About) would be interested in. Of
course the updater runs on the main thread so we’re proxying them
through IPC. It’s fine.

src/application-delegate.coffee

@@ -182,6 +182,34 @@ class ApplicationDelegate
     new Disposable ->
       ipcRenderer.removeListener('message', outerCallback)
 
+  onDidBeginCheckingForUpdate: (callback) ->
+    outerCallback = (message, detail) ->
+      if message is 'checking-for-update'
+        callback(detail)
+
+    ipc.on('message', outerCallback)
+    new Disposable ->
+      ipc.removeListener('message', outerCallback)
+
+  onDidCompleteDownloadingUpdate: (callback) ->
+    outerCallback = (message, detail) ->
+      if message is 'update-downloaded'
+        callback(detail)
+
+    ipc.on('message', outerCallback)
+    new Disposable ->
+      ipc.removeListener('message', outerCallback)
+
+  onUpdateNotAvailable: (callback) ->
+    outerCallback = (message, detail) ->
+      if message is 'update-not-available'
+        callback(detail)
+
+    ipc.on('message', outerCallback)
+    new Disposable ->
+      ipc.removeListener('message', outerCallback)
+
+
   onApplicationMenuCommand: (callback) ->
     outerCallback = (event, args...) ->
       callback(args...)