feat(MdDialog): add border-radius to <md-dialog-container>

[rolandjitsu]

Bug, feature request, or proposal:

Feature

What is the expected behavior?

<md-dialog-container> should probably have a 2px border radius like the rest of the components.

What is the current behavior?

There is no border radius.

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: http://plnkr.co/edit/o077B6uEiiIgkC0S06dd

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

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

2.0.0-alpha.10

Is there anything else we should know?

[angular-automatic-lock-bot]

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