fix(mdButton): Set default ariaLabel to the elements text

`md-button` was not setting a default `ariaLabel` like the [docs](https://material.angularjs.org/#/api/material.components.button/directive/mdButton) say it does. This fixes #738     if we want `ariaLabel` to be semi optional. Where it will only warn you if an `md-button` doesn't have an `ariaLabel` and the element contains no text. For example if your button is just an icon but you forget to set the `ariaLabel`.