Allow creation of new validators on the fly

This is useful for extensions and/or behaviors.
I could do it outside the model, but I think it belongs in it.
Update changelog
now return $this allowing method chaining

also added @since tag
fixed log order
new line after method

just to keep the standard
Added test for CModel::addValidator method
Better test for CModel::addValidator
Removed extra spaces
Enh #1928: Gii is now able to use table columns' comments as the attribute labels of a new generated model.
Enhancements to the #2277.

1. Change log line fixed.
2. Attribute names fallback mode: column comments are preferred over column names.
3. Related view altered.
Checkbox state checking forgotten.
Merge pull request #2277 from resurtm/gii-column-comments-as-labels

Enh #1928: Gii is now able to use table columns' comments as the attribute labels of a new generated model.
Merge pull request #2259 from gusnips/patch-2

Allow creation of new validators on the fly
 Test case "CModelTest::testAddValidator()" has been fixed.