fixed declaration of Doctrine_Connection_Pgsql::modifyLimitQuery and Doctrine_Connection_Common::modifyLimitQuery to be the same

Removed getState() and concat() methods from mysql driver, getState() is not needed (it resides at transaction class) and concat is located at mysql expression driver

Ticket: 199

Removed some remainders from the old validation behaviour. This fixes a bug where records dont get inserted.

- validators codes are now string instead of integers (constants)
- validating is now handled in record level
- added new component Doctrine_Validator_ErrorStack
- new methods Doctrine_Record::isValid() and Doctrine_Record::getErrorStack()
- errorStack now as a record instance variable

Major change on how transactions are being handled: insert / update queries are now executed immediately and only deletes are being gathered (due to delete optimization strategies). Fixes #138, #135

Ticket: 134