add the symfony DIC interface
register the symfony DIC component in the composer.json
implement container::set container::has and container::setParameter
add missing scope interface
introduce a concept of a frozen container and fixes some tests
Merge branch '7.x-1.x' into symfony-container-interface
fix the test coverage
document frozen
Respect the ContainerInterface::get contract for NULL_ON_INVALID_REFERENCE and not return FALSE
Fix the behaviour for not existing services with NULL_ON_REFERENCE.

* Don't look up the service twice, but also throw an exception if the second call
  specifies EXCEPTION_ON_REFERENCE.
Merge pull request #16 from dawehner/container-get-null

Respect the ContainerInterface::get contract for NULL_ON_INVALID_REFERENCE and not return FALSE