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.

Feature was requested in yiisoft/yii#765

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

MSSQL: select what is really needed from information_schema.columns, not just everything as it was before.