Use MR_defaultContext for all CoreData operations

A shared context via the AppDelegate should not be used
because it adds unnecessary coupling to the appdelegate
and causes tests to break because the tests do not have
access to a UIApplication instance.

Also, as per the MagicalRecord docs, MR_contextForCurrentThread
should not be used on the main thread because MR_contextForCurrentThread
returns the same context as MR_defaultContext in that instance.
See:
  - https://github.com/magicalpanda/MagicalRecord/issues/566
  - http://stackoverflow.com/questions/17814656/passing-a-managedcontext-with-core-date-magical-record