Properly deliver events back to objective-c

This was bugged, as HandlerConverter would be destroyed as soon as its
scope was exited, causing no events to be delivered. By adding an
associated object on the subscription that should be held onto by
objective-c, it now only gets destroyed when the subscription does.

Fixes #7.

Author: richardjrossiii

Sources/ParseLiveQuery/ObjCCompat.swift

@@ -254,26 +254,29 @@ extension Client {
     private class HandlerConverter: SubscriptionHandling {
         typealias PFObjectSubclass = PFObject
 
-        private let handler: ObjCCompat_SubscriptionHandling
+        private static var associatedObjectKey: Int = 0
+        private weak var handler: ObjCCompat_SubscriptionHandling?
 
         init(handler: ObjCCompat_SubscriptionHandling) {
             self.handler = handler
+
+            objc_setAssociatedObject(handler, &HandlerConverter.associatedObjectKey, self, .OBJC_ASSOCIATION_RETAIN)
         }
 
         private func didReceive(event: Event<PFObjectSubclass>, forQuery query: PFQuery, inClient client: Client) {
-            handler.didRecieveEvent?(query, event: ObjCCompat.Event(event: event), client: client)
+            handler?.didRecieveEvent?(query, event: ObjCCompat.Event(event: event), client: client)
         }
 
         private func didEncounter(error: ErrorType, forQuery query: PFQuery, inClient client: Client) {
-            handler.didRecieveError?(query, error: error as NSError, client: client)
+            handler?.didRecieveError?(query, error: error as NSError, client: client)
         }
 
         private func didSubscribe(toQuery query: PFQuery, inClient client: Client) {
-            handler.didSubscribe?(query, client: client)
+            handler?.didSubscribe?(query, client: client)
         }
 
         private func didUnsubscribe(fromQuery query: PFQuery, inClient client: Client) {
-            handler.didUnsubscribe?(query, client: client)
+            handler?.didUnsubscribe?(query, client: client)
         }
     }