Minor improvements to the team creation command

Also remove the keyword `new` from clearing a team as it might be confusing now that teams are created with a different command.
Refactor list teams command to show team sizes and empty teams list
Add support to remove/delete a team (admin user required)
Decorate team_name in messages
Add command header comments for better separation of the script
Support no 'team-name' specified for commands

Use a hidden default team label to store users that don't specify a team-name.
Update README with multi teams commands
Merge pull request #5 from hubot-scripts/multi-teams-followup

Multi teams followup