this allows us to implement other activerecord implementations based on
NoSQL 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

- The dropConstraint method can be used both for dropPrimaryKey and dropForeignKey

Feature was requested in yiisoft/yii#765

finished Sort.