Made SessionDelegate subscript public to allow full control when subclassing.

Source/Manager.swift

@@ -222,7 +222,8 @@ public class Manager {
         private var subdelegates: [Int: Request.TaskDelegate] = [:]
         private let subdelegateQueue = dispatch_queue_create(nil, DISPATCH_QUEUE_CONCURRENT)
 
-        subscript(task: NSURLSessionTask) -> Request.TaskDelegate? {
+        /// Access the task delegate for the specified task in a thread-safe manner.
+        public subscript(task: NSURLSessionTask) -> Request.TaskDelegate? {
             get {
                 var subdelegate: Request.TaskDelegate?
                 dispatch_sync(subdelegateQueue) { subdelegate = self.subdelegates[task.taskIdentifier] }