Add ListCommand spec

This also makes the list of commands available through Command.all
Add .travis.yml and build PNG, update contribution README

Refs #19
Introduce EnvCommand triggered by '?'

Refs #21. This makes the REPL less verbose and only prints environment info when asked for.
Change gem name to 'replicant-adb' to avoid name clash

Refs #22
Update README
Badge all-the-things