Finished ActiveDataProvider.

- 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

[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(...)

MSSQL seems to support limits for decimal type, so we add a default
limit.