Refactor to use ngAria

This PR incorporates @btford's commits from https://github.com/angular/material/pull/316 and relates to https://github.com/angular/material/issues/314. 

One outstanding issue with this change: 
 1. `material-switch` needs refactoring to be supported (https://github.com/angular/material/issues/321), ARIA-specific tests have been commented out
Do you think it is a valid and good idea?