2.5 validation method chaining

Following the other recent changes in the API of 2.5, this also makes a method allowing to be chained.
Especially since this is just a wrapper for setRule().

Doctype of setRule() is:

    @return CakeValidationSet this instance

PS: I wonder why this method is needed besides setRule() as it seems both do exactly the same?

Alternatively, a note could be added to state that setRule() is the main method and this only a wrapper for compatibility with ArrayAccess etc.