Adding ConditionalCriteriaVisitor interface which allows `shouldVisit` checking
Skip `visitor->visit` if `ConditionalCriteria->shouldVisit` returns false
OrIdEquals now implements `ConditionalCriteriaVisitor` (id > 0)
Adding `beberlei/assert` and `php_codesniffer` as dependency
Asserting that all visitors are instance of `CriteriaVisitor`
Adding Makefile rule `phpcs` to check CS against PSR-2
Merge pull request #2 from andreyknupp/feature-conditional-criteria-visitor

Adding `ConditionalCriteriaVisitor` for pre-checking purposes