Add mode-setting commands to the CLI.

New commands:
- set_server forces mode to "server"
- set_agent forces mode to "agent"

This makes it more convenient, potentially, to bring up servers
where you want to explicitly configure them as servers, regardless
of how many servers are desired in total.
CLI help output lists available commands

Makes it much more useful.
Merge pull request #8 from ethanrowe/mode-control

Resolves #6 and resolves ethanrowe/auto-consul#7