Add backticks to columns

Fix for searching when you are running this on a table with column names that are reserved words, e.g. 'key' or 'values'. Otherwise you will get a 500 server error.

There is another query around line 570 that I think also needs to be updated but I haven't tested this.
$column = $db_prefix . $columns_clean[$i];

will need to be replaced with

// add backticks to columns, needed in order to work around (mysql) reserved words
$column = explode('.',$db_prefix . $columns_clean[$i]);
foreach ($column as $key => $value)
  $column[$key] = "`".$value."`";
$column = implode('.',$column);