expose DetachedDataTransaction's context and allow creating children detached transactions (https://github.com/JohnEstropia/CoreStore/issues/9)

CoreStore/Saving and Processing/DataStack+Transaction.swift

@@ -62,7 +62,7 @@ public extension DataStack {
     }
     
     /**
-    Begins a non-contiguous transaction where `NSManagedObject` creates, updates, and deletes can be made. This is useful for making temporary changes, such as partially filled forms. A detached transaction object should typically be only used from the main queue.
+    Begins a non-contiguous transaction where `NSManagedObject` creates, updates, and deletes can be made. This is useful for making temporary changes, such as partially filled forms.
     
     - returns: a `DetachedDataTransaction` instance where creates, updates, and deletes can be made.
     */
@@ -70,6 +70,10 @@ public extension DataStack {
         
         return DetachedDataTransaction(
             mainContext: self.rootSavingContext,
-            queue: .Main)
+            queue: .createSerial(
+                "com.coreStore.dataStack.detachedTransactionQueue",
+                targetQueue: .UserInitiated
+            )
+        )
     }
 }

CoreStore/Saving and Processing/DetachedDataTransaction.swift

@@ -36,6 +36,18 @@ public final class DetachedDataTransaction: BaseDataTransaction {
     
     // MARK: Public
     
+    /**
+    Returns the `NSManagedObjectContext` for this detached transaction. Use only for cases where external frameworks need an `NSManagedObjectContext` instance to work with.
+    
+    Note that it is the developer's responsibility to ensure the following:
+    - that the `DetachedDataTransaction` that owns this context should be strongly referenced and prevented from being deallocated during the context's lifetime
+    - that all saves will be done either through the `DetachedDataTransaction`'s `commit(...)` method, or by calling `save()` manually on the context, its parent, and all other ancestor contexts if there are any.
+    */
+    public var internalContext: NSManagedObjectContext {
+        
+        return self.context
+    }
+    
     /**
     Saves the transaction changes asynchronously. For a `DetachedDataTransaction`, multiple commits are allowed, although it is the developer's responsibility to ensure a reasonable leeway to prevent blocking the main thread.
     
@@ -50,6 +62,19 @@ public final class DetachedDataTransaction: BaseDataTransaction {
         }
     }
     
+    /**
+    Begins a child transaction where `NSManagedObject` creates, updates, and deletes can be made. This is useful for making temporary changes, such as partially filled forms.
+    
+    - returns: a `DetachedDataTransaction` instance where creates, updates, and deletes can be made.
+    */
+    public func beginDetached() -> DetachedDataTransaction {
+        
+        return DetachedDataTransaction(
+            mainContext: self.context,
+            queue: self.transactionQueue
+        )
+    }
+    
     
     // MARK: Internal