fixes in join statements + handling of NOT predicate

Hi

My first commit adds handling for NOT predicate.
The second one is a fix in conversion of joinStatementsArray to NSString. There was a comma separator used for this, but this produced not well formed SQL statement.
The third commit fixes join statement construction for deeper relationships. The problems was that 'currentEntity' and 'lastTableName' were not updated to reflect the current level of relationship, resulting in references to root level entity/table name on each level.

Best regards
Rafal