Better flexibility for abstract db types

Feature was requested in yiisoft/yii#765
Added phpdoc about precision constraints for abstract db types
Sqlite unit test for abstract db types
Adjusted type map in MSSQL Querybuilder

MSSQL seems to support limits for decimal type, so we add a default
limit.
Merge pull request #477 from cebe/querybuilder-dynamic-column-types

[READY] Better flexibility for abstract db types