Correctly export EventStore as property of itself

Makes this work:

  const { EventStore, Consumer } = requre('event-storage');

index.js

@@ -1,4 +1,5 @@
-module.exports = module.exports.EventStore = require('./src/EventStore');
+module.exports = require('./src/EventStore');
+module.exports.EventStore = module.exports;
 module.exports.EventStream = require('./src/EventStream');
 module.exports.Storage = require('./src/Storage');
 module.exports.Index = require('./src/Index');