fixes #854

this allows different implementation in different DBMS

CUBRID does not supprt PDO::TYPE_BOOL, so we use STRING here which will
be casted by the DBMS

- Added drop/add primary key methods to QueryBuilder.php
- Added mysql specific dropPrimarykey method
- Added sqlite specific dropPrimarykey and addPrimaryKey methods
- Added uint testing for dropPrimarykey and addPrimaryKey methods
- Corrected postgresql column types, by adding length and precision

PHP supports exception stacks since 5.3 so we should use it.
Also Errorhandler is now able to display the stack: #297