Switched the TaskDelegate data and error properties over to a public ACL.

Source/TaskDelegate.swift

@@ -33,13 +33,16 @@ open class TaskDelegate: NSObject {
     /// The serial operation queue used to execute all operations after the task completes.
     open let queue: OperationQueue
 
+    /// The data returned by the server.
+    public var data: Data? { return nil }
+
+    /// The error generated throughout the lifecyle of the task.
+    public var error: Error?
+
     var task: URLSessionTask? {
         didSet { reset() }
     }
 
-    var data: Data? { return nil }
-    var error: Error?
-
     var initialResponseTime: CFAbsoluteTime?
     var credential: URLCredential?
     var metrics: AnyObject? // URLSessionTaskMetrics