Support multiple API tokens

[dblock]

Ala https://github.com/exciting-io/slack-bot-server, although Slack is saying they are working on providing a better method than running actual multiple bots.

[dominicsayers]

👍

[dblock]

Related, I built a proof of concept for running a bot for multiple teams in https://github.com/dblock/slack-bot-server.

A few things had to change in slack-ruby-bot:

• Split App into Server + App, dblock@5e54447
• Make tokens local to a client instance, dblock@da76192
• Refactor all globals that can now be defined via config and locally to a server, dblock@181327c

However we probably still want to integrate a proper service class in slack-ruby-bot that supports multiple tokens.

[dblock]

Closing in favor of https://github.com/dblock/slack-bot-server, and specifically slack-ruby/slack-ruby-bot-server#3.