feat(md-card): add md-card-footer

[Nightapes]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

md-card-footer directive is missing.

@Directive({ selector: 'md-card-footer' }) export class MdCardFooter { }

What is the current behavior?

zone.js?1473057816570:484 Unhandled Promise rejection: Template parse errors: 'md-card-footer' is not a known element:

When using footer, the directive is not found

What are the steps to reproduce?

Add md-card-footer to the demo-app

What is the use-case or motivation for changing an existing behavior?

For adding a progress-bar add the bottom:
https://material.google.com/components/progress-activity.html#progress-activity-behavior

Which versions of Angular, Material, OS, browsers are affected?

Angular 2.0.0 RC.6 and Material2 alpha.8

Is there anything else we should know?

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Nightapes]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[hansl]

@kara could you review this change?

[hansl]

The rest looks good, just want to make sure the style changes are good.

[kara]

LGTM

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