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.
Specs for 44ed75af23aca2d67e551eefe3fc11e37e2d26e9
Merge #156
Add CHANGELOG entry for #156, add nil checks for container matching methods
Add Serverspec tests for busybox-container 7777/8888
Merge branch 'fancyremarker-named-containers'