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. 

Two outstanding issues with this change: 
 1. `material-switch` needs refactoring to be supported (https://github.com/angular/material/issues/321) 
 1. `material-checkbox` does not receive expected attribute `aria-checked` despite binding to `ngModel`