Make tooltip examples accessible

This is a temporary accessibility fix for `<md-tooltip>`. Because of an outstanding accessibility issue with `<md-icon>` (https://github.com/angular/material/issues/427), `<md-button>` components used in conjunction with `<md-tooltip>` do not have accessible names and are read aloud in a screen reader simply as "button". 

The current fix is to add `aria-label` to the parent button element, but the ideal solution would be to require `<md-icon>` to label itself.