aper-radio-group` allows user to select only one radio button from a set.
Checking one radio button that belongs to a radio group unchecks any 
previously checked radio button within the same group. Use
`selected` to get or set the selected radio button.

Example:

    <paper-radio-group selected="small">
      <paper-radio-button name="small" label="Small"></paper-radio-button>
      <paper-radio-button name="medium" label="Medium"></paper-radio-button>
      <paper-radio-button name="large" label="Large">