Changes in \KsUtils\Validator class:
* Added $errorMessage variable
* getError() method changed from abstract to concrete
* Apidoc added
Changes in \KsUtils\Validator\Date class:
* Added $errorMessage variable definition moved from method to contructor
* Apidoc added
Changes in \KsUtils\Validator class: added added setter for $errorMessage variable
Added tests for  \KsUtils\Validator\Date class
Refactor: 
* Changed method \KsUtils\Validator::setErrorMessage() to setError()
* Changed class variable name from \KsUtils\Validator::errorMessage to error