Create a new pass.  template  - The template fields    - Pass fields (description, serialNumber, logoText)
function Pass(template, fields, images) {
  this.template = template;
  this.fields = cloneObject(fields);
  // Structure is basically reference to all the fields under a given style
  // key, e.g. if style is coupon then structure.primaryFields maps to
  // fields.coupon.primaryFields.
  var style = template.style;
  this.structure = this.fields[style];
  if (!this.structure)
    this.structure = this.fields[style] = {};
  this.images = cloneObject(images);
}