SelectField doesn't work inside Dialog

The drop downs are rendered underneath the dialog and are invisible.

```jsx
import { Dialog, DialogContent } from 'react-mdl';
import { SelectField, Option } from 'react-mdl-extra';

export default (props) => (
  <Dialog open={props.open}>
    <DialogContent>
      <SelectField>
        <Option value={1}>One</Option>
      </SelectField>
    </DialogContent>
  </Dialog>
);
```