ariaLabel defaults

[bdc]

Since v0.5, I am getting a lot of warnings in the console about aria-label:
ARIA: Attribute "aria-label", required for accessibility, is missing on "md-button.md-raised.md-primary.md-default-theme"
ARIA: Attribute "aria-label", required for accessibility, is missing on "md-checkbox.ng-pristine.ng-untouched.ng-valid" .

The docs at material.angularjs.org say this attribute is optional, and will use a button's or checkbox's text as a default.

[ThomasBurleson]

Some of these issues may be resolved with changes in PR 558.

[marcysutton]

I think I fixed some in #512 too.