Md-toast should be accessible

[marcysutton]

One of the more complex interactions, <md-toast>, needs to be made accessible. Because this is an interim element like <md-dialog>, it shares common characteristics and code, with the exception that md-toast goes away automatically after a timeout.

What needs to happen with toast:

1. Focus should be sent to the toast or toast child action for keyboard access
2. Toasts should be read aloud in a screen reader when they appear, either by using aria-live on a parent element or by sending focus to it
3. Focus should be sent back to triggering element after toast disappears/closes.

Sending focus to the element seems to make the most sense, since adding aria-live to the toast's parent element puts responsibility outside of the component itself. Adding tabindex="-1" will make the toast focusable by script.

[marcysutton]

Oops! This is a duplicate of #349.