Adding support for mapping child attribute to a parent attribute

If you have a class like this:

```objective-c

@interface Person : NSObject

@property (nonatomic, copy)   NSString *name;
@property (nonatomic, copy)   NSString *email;
@property (nonatomic, copy)   NSString *skype;

@end


```

And a JSON like this:

```JSON

{
    "person": {
        "name": "Lucas",
        "email": "lucastoc@gmail.com",
        "contact": {
            "skype": "aspmedeiros"
        }
    }
}

```

You can map it like this

```objective-c

[BWObjectMapping mappingForObject:[Person class] block:^(BWObjectMapping *mapping) {
                [mapping mapKeyPath:@"name" toAttribute:@"name"];
                [mapping mapKeyPath:@"email" toAttribute:@"email"];
                [mapping mapKeyPath:@"contact.skype" toAttribute:@"skype"];
                [[BWObjectMapper shared] registerMapping:mapping withRootKeyPath:@"person"];
            }];

```