Make push follow controller and adapter style

src/Adapters/Push/PushAdapter.js

@@ -3,27 +3,15 @@
 // Allows you to change the push notification mechanism.
 //
 // Adapter classes must implement the following functions:
-// * initialize(pushConfig)
-// * getPushSenders(parseConfig)
-// * getValidPushTypes(parseConfig)
+// * getValidPushTypes()
 // * send(devices, installations)
 //
 // Default is ParsePushAdapter, which uses GCM for
 // android push and APNS for ios push.
+export class PushAdapter {
+  send(devices, installations) { }
 
-var ParsePushAdapter = require('./ParsePushAdapter');
-
-var adapter = new ParsePushAdapter();
-
-function setAdapter(pushAdapter) {
-  adapter = pushAdapter;
+  getValidPushTypes() { }
 }
 
-function getAdapter() {
-  return adapter;
-}
-
-module.exports = {
-  getAdapter: getAdapter,
-  setAdapter: setAdapter
-};
+export default PushAdapter;