checkbox group

#### Bug, feature request, or proposal:
Feature request

#### What is the expected behavior?
Similar to the multiple select, it should be possible to have a list of check boxes to select multiple options returned in a single array of choices. A required validator would check for at least one selection.

#### What is the current behavior?
Has to be made via select or custom logic.

#### What are the steps to reproduce?

Providing a Plunker (or similar) is the *best* way to get the team to see your issue.
Plunker template: https://goo.gl/DlHd6U

#### What is the use-case or motivation for changing an existing behavior?
Need it quite often for my projects. Sometimes a select drop down is not what customers wish for a multiple selection. Sometimes it is better to see all options without having to open a menu.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Ng4.1.x, MD beta 6

#### Is there anything else we should know?

まず、既存の複数選択コンポーネントとチェックボックスコンポーネントを確認し、現在のAPIとバリデーションの動作を理解します。チェックボックスグループの動作を、複数の選択済みオプションを返し、少なくとも1つの選択を必須とするように定義します。これらのインタラクションとバリデーションが一貫して機能すれば、issueは完了です。

索引モデルが issue の本文から書いたものです。