roup Paper Elements

`paper-button` is a button containing text or an image. When the user touches
the button, a ripple effect emanates from the point of contact.

A `paper-button` may be flat or raised. A raised button behaves like a piece
of paper resting on another sheet, and lifts up upon press. Flat buttons do
not raise up. Add the `raisedButton` attribute to make a raised button.

Example:

    <paper-button label="flat button"></paper-button>
    <paper-button label="raised button" raisedButton></pap