Introduce RKPropertyMapping abstract superclass for RKAttributeMapping and RKRelationshipMapping.

Add primitives for working with attribute, relationship, and property mapping objects.

Code/ObjectMapping/RKPropertyMapping.h

@@ -0,0 +1,54 @@
+//
+//  RKPropertyMapping.h
+//  RestKit
+//
+//  Created by Blake Watters on 8/27/12.
+//  Copyright (c) 2009-2012 RestKit. All rights reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+#import <Foundation/Foundation.h>
+
+/**
+ RKPropertyMapping is an abstract class for describing the properties being mapped
+ within an object or entity mapping. It defines the common interface for its concrete
+ subclasses RKAttributeMapping and RKRelationshipMapping.
+ */
+@interface RKPropertyMapping : NSObject <NSCopying>
+
+/**
+ A key path on the source object from which to get information that is to be mapped onto
+ the destination object.
+ */
+@property (nonatomic, strong, readonly) NSString *sourceKeyPath;
+
+/**
+ A key path on the destination object on which to set information that has been mapped from
+ the source object.
+ */
+@property (nonatomic, strong, readonly) NSString *destinationKeyPath;
+
+/**
+ Compares the receiving property mapping to another property mapping.
+ 
+ Two property mappings are equal if they are of the same type (i.e. an attribute or a
+ relationship mapping) and specify a mapping from the same source key path to the 
+ same destination key path.
+ 
+ @param otherMapping The property mapping object with which to compare the receiver.
+ @return YES if otherMapping specifies the same mapping as the receiver, otherwise NO.
+ */
+- (BOOL)isEqualToMapping:(RKPropertyMapping *)otherMapping;
+
+@end