Support Nested Serialization

I have a managed object mapping that maps attributes like this:
```
[mapping mapFieldsFromDictionary:@{
    @"id":@"serverID",
    @"info.title":@"title",
    @"info.createDate":@"createDate",
}];
```

The corresponding Dictionary looks like this:
```
{
    @"id":@"12345",
    @"info":@{
        @"title":@"title",
        @"createDate":@"2013-05-23"
    }
};
```

EKMapper executes just as expected and parses the nested dictionary correctly.
Serializing the resulting object with EKSerializer however ends up generating a Dictionary that looks like this:

```
{
    @"id":@"12345",
    @"info.title":@"title",
    @"info.createDate":@"2013-05-23"
}
```

The value with ```[representation setObject:reverseValue forKey:fieldMapping.keyPath]``` path in ```setValueOnRepresentation:fromObject:withFieldMapping:```.

Better would be to split the keypath like ```NSArray *keyPathComponents = [keyPath componentsSeparatedByString:@"."];``` and enumerate over the components.