Exported Tier events from tiers package

These are required to be exported so that external code can subscribe to the
events.

ghost/tiers/index.js

@@ -1,5 +1,16 @@
+const TierActivatedEvent = require('./lib/TierActivatedEvent');
+const TierArchivedEvent = require('./lib/TierArchivedEvent');
+const TierCreatedEvent = require('./lib/TierCreatedEvent');
+const TierNameChangeEvent = require('./lib/TierNameChangeEvent');
+const TierPriceChangeEvent = require('./lib/TierPriceChangeEvent');
+
 module.exports = {
     Tier: require('./lib/Tier'),
     TiersAPI: require('./lib/TiersAPI'),
-    InMemoryTierRepository: require('./lib/InMemoryTierRepository')
+    InMemoryTierRepository: require('./lib/InMemoryTierRepository'),
+    TierActivatedEvent,
+    TierArchivedEvent,
+    TierCreatedEvent,
+    TierNameChangeEvent,
+    TierPriceChangeEvent
 };