Create a new template.  style  - Pass style (coupon, eventTicket, etc) fields - Pass fields (passTypeIdentifier, teamIdentifier, etc)
function Template(style, fields) {
  if (!~STYLES.indexOf(style))
    throw new Error("Unsupported pass style " + style);
  this.style = style;
  this.fields = {};
  for (var key in fields)
    this.fields[key] = fields[key];
  this.keysPath = "keys";
  this.images = {};
}