Changed contexts merging logic from SaveWithBlock methods

Merge all changes from Root saving Context in default context can cause unwanted behaviours if the changes are initilally generated in default context itself (Default Context -> Root Context -> Default context). 
In many cases - especially on iOS 6 - deleted MO “reappears” in the default context after saving
with MR_saveToPersistantStore methods.
For this reason I’ve removed the observer from the root context and added an observer only for localcontext created in saveWithBlock methods