Setting services in the constructor

[alister]

With the move in Symfony to defaulting all services to private, unless explicitly set public, using code like $this->getContainer()->get('dz.slack.users'); to get the services within the code is also somewhat obsolete.

By registering commands as services (see #28) it is also now better to inject any services into the constructor wherever possible rather than extending ContainerAwareCommand (extend Symfony\Component\Console\Command\Command).