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
Merge pull request #2259 from gusnips/patch-2

Allow creation of new validators on the fly