Rename "noun and verb" to "command and subcommand"

The noun and verb metaphor provided a good starting point for organizing the client commands, but it is currently stretched and it no longer seems constructive to fit things into this paradigm. For example, we have one verb called 'status', which is not a verb. Rather than try to force things to be nouns and verbs, rename Noun to Command and Verb to Subcommand.