perf(): use ngDisabled internally instead of disabled

Closes #607.

BREAKING CHANGE: For performance, the ng-disabled attribute is now read
to check if a component is disabled instead of the disabled attribute.

If you use the `disabled` attribute on a component to set whether
it is disabled, change it to an ng-disabled expression.

Change your code from this:

```html
<md-checkbox disabled></md-checkbox>
```

To this:

```html
<md-checkboxn ng-disabled="true"></md-checkbox>
```