Add support for excerpt projections

When representations are rendered in HAL, we currently render the complete entity in the _embedded clause for collection resources. If an item resource is accessed, we currently don't render an _embedded clause. This ticket suggests to allow to customize this behavior as follows.

• the user has means to identify a projection interface (see DATAREST-221 for the general mechanism) as an excerpt projection
• for collection resources the excerpt will automatically rendered in _embedded clauses if an excerpt projection is in place
• for related resources, the excerpt projection will be rendered. If the value is rendered within an _embedded clause itself, the links to the related resource shall be omitted.

Example

class Author {
    public String name;
    public Set<Book> books;
}

class Book {
    public String title;
    public Set<Author> authors;
}

interface BooksExcerpt {
    String getTitle();
}

authors collection resource - full rendering of items but related resource's excerpt rendered embedded.

{ _embedded : {
    authors : [{
      name : "Oliver Gierke",
      _embedded : {
        books : [{
          title : "Spring Data"
        }]
      },
      _links : {
        self : { href : "…" }
      }
    }]
  }
}

author item resource - full rendering of item, related resource's excerpt rendered embedded, link to related resource present

{ name : "Oliver Gierke",
  _embedded : {
    books : [{
      title : "Spring Data"
    }]
  },
  _links : {
    self : { href : "…" },
    books : { href : "…" }
  }
}