Support command line interface for mailman

Hello, I've added an executable file and conventions/configurations for deploying Mailman in production environment. Here are what this PR adds:

## Executable file

    `mailman -c [config file] -p [pid file] -e [environment]`

Config file contains configurations and instructions for Mailman, like `mailman_app.rb` in `USER_GUIDE.md` does. It should be provided in order to execute `mailman`. Pid file and environment parameters are optional.

## Capistrano recipe

If you use Capistrano to deploy your application, just add

    require 'mailman/capistrano`

in your `deploy.rb`.