paper-button-group allows user to select only one button from a set.
Checking one button that belongs to a group unchecks any
previously checked button within the same group. Use
selected to get or set the selected button.
Example:
<paper-button-group selected="2">
<paper-button>Oxygen</paper-button>
<paper-button>Carbon</paper-button>
<paper-button>Hydrogen</paper-button>
<paper-button>Nitrogen</paper-button>
<paper-button>Calcium</paper-button>
</paper-button-group>