##<paper-date-input>
paper-date-input is a single-line text field with Material Design styling modified from gold-cc-expiration-input
It only supports YYYY-MM-DD format for now.
The input can check whether the entered date is a valid, future date.
The input can be automatically validated as the user is typing by using
the auto-validate and required attributes. For manual validation, the
element also has a validate() method, which returns the validity of the
input as well sets any appropriate error messages and styles.
See Polymer.PaperInputBehavior for more API docs.
See Polymer.PaperInputContainer for a list of custom properties used to
style this element.