Add inContext variant for `+MR_propertiesNamed:`

Closes #773.
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	MagicalRecord/Categories/NSManagedObject/NSManagedObject+MagicalRecord.h
	MagicalRecord/Categories/NSManagedObject/NSManagedObject+MagicalRecord.m
Update the docs regarding the saving to explain the problem related to MR_contextForCurrentThread and saveWithBlock