Added NSManagedObjectContext+ChainSave method just like the [MagicalRecord saveWithBlock…] methods, this set of methods start with the receiver context and creates a child context. Upon saving, the child context saves to parent and chain up the save operation to main context, and finally to saving context. This is to ensure that all changes made in background will pass through to main context and then to persistent store.
Added unit test
Updated method names
Updated unit test
Merge remote-tracking branch 'ma/develop' into develop

Conflicts:
	MagicalRecord.xcodeproj/xcshareddata/xcschemes/libMagicalRecord-OSX.xcscheme
	MagicalRecord.xcodeproj/xcshareddata/xcschemes/libMagicalRecord-iOS.xcscheme
Merge pull request #843 from callzhang/develop

Added methods to support chained saves on NSManagedObjectContext