parse(containerDocument) {
    //-- <rootfile full-path="OPS/package.opf" media-type="application/oebps-package+xml"/>
    var rootfile;

    if (!containerDocument) {
      throw new Error("Container File Not Found");
    }

    rootfile = Object(core["qs"])(containerDocument, "rootfile");

    if (!rootfile) {
      throw new Error("No RootFile Found");
    }

    this.packagePath = rootfile.getAttribute("full-path");
    this.directory = path_default.a.dirname(this.packagePath);
    this.encoding = containerDocument.xmlEncoding;
  }
Parse the Container XML
@param  {document} containerDocument