Improved checks where substr_compare is used, replaced more substr + comapre cases with substr_compare

PR #3676

Alternatively, if statement could be: if(substr($column->dbType, 0, 4) === 'enum'), however expected implemented option to be more robust across other database formats.

Created after followup on this issue:  https://github.com/yiisoft/yii/issues/292#issuecomment-44780779

Adding a new additional validation rule in the gii-crud generator enforcing the expected format.

meet the AR interface requirements.

now supports mapped primary key when _id is part of source with an
alias.