Fix for saving from defaultContext in ios 6

Merging all changes in root context into default context can cause
errors if the changes are started from defaultContext (defaultContext
-> root context -> default context). I’ve observed - especially in iOS
6 - that many times after merging the changes from the root into
default context the mo are kept into the default context in the deleted
state.
I’ve removed the observer from the root cotext and I’ve added an
observer only for the local contexts  in the saveWithBlock methods. In
this way the changes are persisted off (main)thread and the main
context is updated almost immediately