Synchronized import of arrays

This way you can delete all the elements in the database of the importing entity, except the ones you are importing now. 

Can be useful when using LFSCoreData as a cache of your data and this data is volatile.

The usage is like this: 

	[Tweet importFromArray:yourArrayOfNewObjects
	             inContext:[[LFSDataModel sharedModel] mainContext]
	    deleteOtherObjects:YES];