#205 skip attribute validation, if min => 0 & input not exist

[DaveFerger]

is have an opened issue #205

this fixed it:
on form beforeValidateAttribute event we check the attribute's input is exist,
if not exist we return false, so we dont validate this attribute

otherwise:
form validation have error on not exist element
so the form submitted(yiiActiveform.js don't find form-group with has-error class), but the errorSummary updated to and show errors for non-exist inputs