Fix crash when signing in with a self-hosted blog

- The blog needed a permanent ID for Categories and likely the rest of its request entities
Use existingObjectWithID for Comments

- Again with iOS 6 being overly sensitive

- performBlock: for merging posts and pages
Add a completion block for non-blocking saves
Fix issue with post status update after saving

- The derived context falls out of scope by the time the upload is underway and completes.

- Translate the object to the main context for further UI updates.

- Keep performance of closing the edit view by using a completion block for performing the save.

- Remove subclass redeclaration of uploadWithSuccess:failure: