Retain cycle causes ARBase and ARRelationship leaks

ARBase retains its relationships but each relationship also retains its parent 'record' creating a retain cycle.

Change the record property of ARRelationship to a weak reference (c.f. delegates).

diff --git a/Source/Relationships/ARRelationship.h b/Source/Relationships/ARRelationship.h
index 04f9753..6d8d73c 100755
--- a/Source/Relationships/ARRelationship.h
+++ b/Source/Relationships/ARRelationship.h
@@ -51,7 +51,7 @@
   ARBase *record;
 }
 @property(readwrite, retain) NSString *name, *className;
-@property(readwrite, retain) ARBase *record;
+@property(readwrite, assign) ARBase *record;
 
 /*!
  * Creates a relationship with a given name using the passed class