Reverted 0bf2dadd: it's very common to use lower case for column types and upper case for other DB keywords.

- 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

Feature was requested in yiisoft/yii#765