Skipping exceptions

Added a couple of things:

- removed composer.lock since this is a library
- added phpunit and it's executable to composer for easier testing
- extracted from `Errbit` the code responsible for writing to sockets to a `SocketWriter` class
- thanks to extracting `SocketWriter` I was able to mock it and write a test for `Erbit`
- added exceptions skipping. Use case: I use `FOSRestBundle` for RESTful API and when I throw for example a `ConflictHttpException` which generates a response with status code `409`. This is intended behavior and I don't want it to be logged be Errbit. Thus I can specify `ConflictHttpException` (FQDN of course) in the `skipped_exceptions` configuration parameter.