See the following resources for details:
yiisoft/yii#779
https://bugs.php.net/bug.php?id=33876
http://www.yiiframework.com/forum/index.php/topic/32334-boolean-type-with-postgresql/page__view__findpost__p__155647

Relates #1086 and #1087

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

[2] Added empty line at the end of file
[3] Removed exception.

Removed unused columsn from find constraint sql. Fixed typo. Added extra schema check for when a foreign table is not in the same schema. Updated indentation to conform to other classes.

Refactored == null to is_null == '' to empty(...)