SPJSONStorage: Switches over to properties
SPStorageProvider: Adds new sync helper
SPJSONStorage: Implements new sync helper
SPCoreDataStorage: Implements new sync helper
Simperium: Updates saveWithoutSyncing logic
Prevents multiple logout calls
Simperium: completes signout after the writerMOC finishes
Merge pull request #413 from Simperium/issue/411-handles-async-save-during-logout

Handles async Save-OP during logout