Link ReaderPost and Note to Account

- Deletion of Account is cascaded to ReaderPost and Note

- Remove manual deletions
Improve sign out performance + side effects

- A few bugs arose with iOS 6:
  - The sidebar reloads from the notification and numberOfSections reports the wrong number during the delete. Remove extra reload
  - Blogs are accessed and try to mutate during delete, prevented with checking if object is deleted first

- Set the static default account on the main context for UI usage, regardless of what context it was created on. In getting the default account above, the static is referenced or a main context reference saved off. Keeping it consistent.

- Create the account on a background context
Change Core Data to dual singleton contexts

- Background context is NSPrivateQueueConcurrencyType

- Main context is NSMainQueueConcurrencyType

- Both point to persistent store coordinator

- On background save, merges into main. On main save, merges into background.

- Solves any issues with derived contexts falling out of scope during usage. Improves simplicity of storing data.

- Refactor Note syncing to use the background context properly

#344
Request the count for synced items instead of all

- For determining the batch size for posts/pages.
Fixes various issues for #344

- Hanging happens when combining performBlock: and performBlockAndWait: mitigating the hangs in favour of a little performance hit right now. Doesn't impact performance too much.

- Saving a context is more generic

- Temporarily using the main context for post editor again to have it work. WIP.

- Fix for creating jetpack account when using blog off the main context or the background context.

This commit is suitable for Pivotal Tracker for stories above the week 21 marker.