function htmlSanitizeWriter(buf, uriValidator){
  var ignore = false;
  var out = angular.bind(buf, buf.push);
  return {
    start: function(tag, attrs, unary){
      tag = angular.lowercase(tag);
      if (!ignore && specialElements[tag]) {
        ignore = tag;
      }
      if (!ignore && validElements[tag] === true) {
        out('<');
        out(tag);
        angular.forEach(attrs, function(value, key){
          var lkey=angular.lowercase(key);
          var isImage = (tag === 'img' && lkey === 'src') || (lkey === 'background');
          if (validAttrs[lkey] === true &&
            (uriAttrs[lkey] !== true || uriValidator(value, isImage))) {
            out(' ');
            out(key);
            out('="');
            out(encodeEntities(value));
            out('"');
          }
        });
        out(unary ? '/>' : '>');
      }
    },
    end: function(tag){
        tag = angular.lowercase(tag);
        if (!ignore && validElements[tag] === true) {
          out('</');
          out(tag);
          out('>');
        }
        if (tag == ignore) {
          ignore = false;
        }
      },
    chars: function(chars){
        if (!ignore) {
          out(encodeEntities(chars));
        }
      }
  };
}
create an HTML/XML writer which writes to buffer
@param {Array} buf use buf.jain('') to get out sanitized html string
@returns {object} in the form of {
    start: function(tag, attrs, unary) {},
    end: function(tag) {},
    chars: function(text) {},
    comment: function(text) {}
}