Make it compatible with container-interop

Use the ContainerInterface from [container-interop][] and add the `has`
method.

[container-interop]: https://github.com/container-interop/container-interop
Test container::has
Merge pull request #3 from joksnet/container-interop

Thanks for the pull request.  Going to also see about what can be done with the exceptions for full interface compatibility.