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?

Start by reviewing the existing multiple-select and checkbox components to understand their current APIs and validation behavior. Define the checkbox-group behavior around returning multiple selected options and requiring at least one selection; the issue is complete when those interactions and validation work consistently.

Written by the indexing model from the issue text.