Issue/1870 Implement Simperium sync

### Notes

- Please, make sure you update your credentials file with Simperium keys.

***Thank you!***

#### Updated CoreData Stack

Simperium is in charge or generating a Background Writer MOC, and wiring up the entire stack. ContextManager's rootContext has been removed.

#### Removed NoteService and NoteServiceRemote

1. There is no need to manually Fetch / MarkAsRead and Refresh notification objects. 
2. Notes should be pushed by Simperium's backend. 
3. Mark as read is handled by simply toggling the 'unread' flag and saving the local MOC.
4. Note objects should be updated by Simperium's backend, when needed.
5. Simperium's backend will keep up to 200 notifications per user. Local prune mechanism has been removed.

#### Updated Podfile + Pods

1. Added Simperium dependency
2. Added a custom origin for SRWebSocket. Original repository has a crash, which has been fixed in the mapped fork. A PR has already been submitted, no news from the developer, yet.

#### Added Data Model v19

1. Renamed Note entity to NoteSimperium, to prevent lightweight migration of legacy entries.
  1. New Attributes simperiumKey / ghostData.
  2. Nuked payload attribute.
  3. Nuked icon attribute.
  4. Nuked account relationship.
  5. Nuked noteID attribute.
  6. Updated subject attribute (Transformable + Optional).
  7. Added body attribute (Transformable + Optional).
  8. Added meta attribute (Transformable + Optional).
2. Updated Account Entity.
  1. Nuked Notes relationship.
3. Added Meta Entity.

Our WordPress-Mobile fork of MGImageUtility has [this Pull Request](https://github.com/wordpress-mobile/MGImageUtilities/pull/1) pending to be merged.

Problem is: we're mapping to that branch's podspec. And [the branch podspec](https://github.com/wordpress-mobile/MGImageUtilities/blob/gifsupport/MGImageUtilities.podspec) source parameter is mapped to the official repository.

This was a temporary fix, i'll be patching our MGImageUtilities fork, thanks for the heads up!