Allow explicit expansion for nested members in projection

This PR addresses #646. Given these destination classes:

```
public class Post
{
    public string Name {get; set;}
    public Author Author {get; set;}
}

public class Author
{
    public string Name {get; set;}
    public ICollection<Post> Posts {get; set;}
}
```

the following calls will expand the Author property but not Name (assuming ExplicitExpansion has been set for both mappings).
```
Project().To<Post>(null, p => p.Author, p => p.Author.Name);
Project().To<Post>(null, "Author", "Author.Name");
```