Adding tests to verify that @usage annotation restrictions work

1. adding tests to verify, that if annotation can't be applied to class/method/property, then approptiate exception is thrown
2. adding descriptive comment around that check to avoid confusion while reading `$usage->$member` code
Merge pull request #75 from php-annotations/67-doubtful-code-in-the-annotationmanager-applyconstrains-method

Fixing retrieval annotations by wrong usage member (not class/method/property)