fix(AccessToken): Move extensions to boot script

Loopback does not facilitate built in model extensions in the same way
as user defined models

server/boot/a-extend-built-ins.js

@@ -0,0 +1,15 @@
+import { Observable } from 'rx';
+
+export default function extendEmail(app) {
+  const { AccessToken, Email } = app.models;
+  Email.send$ = Observable.fromNodeCallback(Email.send, Email);
+  AccessToken.findOne$ = Observable.fromNodeCallback(
+    AccessToken.findOne.bind(AccessToken)
+  );
+  AccessToken.prototype.validate$ = Observable.fromNodeCallback(
+    AccessToken.prototype.validate
+  );
+  AccessToken.prototype.destroy$ = Observable.fromNodeCallback(
+    AccessToken.prototype.destroy
+  );
+}