title: ServiceStack OrmLite LeftJoin Issue
body: <p>I'm using ServiceStack OrmLite JoinSQLBuilder with a left join and have found an issue.
  Suppose I have 2 tables, TableA and TableB and wanted to join on more than a single value.</p>
  
  <p>In SQL I would do something like this:</p>
  
  <pre><code>SELECT  
      TableA.Name, 
      TableB.Value
  FROM    
      TableA
  LEFT JOIN 
      TableB 
  ON     
      TableB.AId = TableA.Id
      AND TableB.PostCode = '12345'  
  </code></pre>
  
  <p>Now the JoinSQLBuilder only allows joins on a single column and generates SQL like so</p>
  
  <pre><code>SELECT  
      TableA.Name, 
      TableB.Value
  FROM    
      TableA
  LEFT JOIN 
      TableB 
  ON     
      TableB.AId = TableA.Id
  WHERE
      TableB.PostCode = '12345'  
  </code></pre>
  
  <p>Which is not the same thing at all!</p>
  
  <p>Is there any way around this in ServiceStack OrmLite?
  Here is an example of left joins with where clauses: <a href="https://stackoverflow.com/questions/4752455/left-join-with-where-clause">Left Join With Where Clause</a></p>