Issue 17

Added internal mapping from (Nodes, Bonds, Actions) to labels.
You created SHNodeLabel along with two other classes SHBondLabel and SHActionLabel. Why do you need 3 of them, while they are identical?

If you need a label represented as a managed object then you just need to create one class SHNodeLabel : NSManagedObject and add '_labelSet' property to SHNode class, as you did it, that's it.