[GH-256] Refactored AnnotationDriver to handle only required classes

This issue is created automatically through a Github pull request on behalf of mattcockayne:

Url: https://github.com/doctrine/common/pull/256

Message:

Whilst debugging an issue I was having I found that the AnnotationDriver uses get_declared_classes() and iterates over the result to identify classes that should be handled.

This seems like a ridiculous overhead especially as it then instantiated a ReflectionClass for each declared class. As an example my current project was iterating over 522 items when it needed to only handle 6.

I've kept the alterations to a minimum but it now takes a snapshot of the declared classes before and after the require for the files and then uses array_diff to get only the classes we are targeting.