Better support for named containers

This modifies container matching to look only at the container name for cases in which a `:container_name` is explicitly provided. The spec I added demonstrates the use case I'm trying to support: namely, deploying newer versions of a tagged image when they become available. Right now, if I pull an updated version of the `busybox` image, I can't redeploy an existing `busybox-container` with the latest image, because the image ID doesn't match.

In particular, I get the following error:

```
STDERR: 2014/05/20 20:52:48 Error: Conflict, The name busybox-container is already assigned to 89efe3432bad. You have to delete (or rename) that container to be able to assign busybox-container to a container again.
```

Does this change jibe with your idea of how container resources should be used?

The kitchen tests passed locally, with the one exception of the `apt-get` failure addressed in #155.