Use instancetype in constructors for increased subclassing friendliness

Code/ObjectMapping/RKObjectMapping.h

@@ -75,7 +75,7 @@
  @param objectClass The class that the mapping targets.
  @return A new mapping object.
  */
-+ (id)mappingForClass:(Class)objectClass;
++ (instancetype)mappingForClass:(Class)objectClass;
 
 /**
  Initializes the receiver with a given object class. This is the designated initializer.
@@ -83,7 +83,7 @@
  @param objectClass The class that the mapping targets. Cannot be `nil`.
  @return The receiver, initialized with the given class.
  */
-- (id)initWithClass:(Class)objectClass;
+- (instancetype)initWithClass:(Class)objectClass;
 
 /**
  Returns an object mapping with an `objectClass` of `NSMutableDictionary`.
@@ -94,7 +94,7 @@
  @see `RKObjectParameterization`
  @see `RKObjectManager`
  */
-+ (id)requestMapping;
++ (instancetype)requestMapping;
 
 ///---------------------------------
 /// @name Managing Property Mappings
@@ -298,7 +298,7 @@
  quickly generate a corresponding serialization mapping from a configured object mapping. The inverse
  mapping will have the source and destination keyPaths swapped for all attribute and relationship mappings.
  */
-- (RKObjectMapping *)inverseMapping;
+- (instancetype)inverseMapping;
 
 ///---------------------------------------------------
 /// @name Obtaining Information About the Target Class
@@ -313,7 +313,6 @@
  @return The class of the property.
  */
 - (Class)classForProperty:(NSString *)propertyName;
-// TODO: Can I eliminate this and just use classForKeyPath:????
 
 /**
  Returns the class of the attribute or relationship property of the target `objectClass` at the given key path.