Rich "Attachment" Notifications Implemented

I've finally implemented Attachment notifications, as suggested  in #24

After a short discussion on IRC with @technicalpickles we've found probably a best way to implement this without extending core hubot functionality. 

I've also wrote an example script that represents, how to use this feature. The code below will result with such message in a chat room:

![screenshot1](http://d.pr/i/uIqC+)

```coffeescript
# Description:
#   Demonstrating Slack Attachments.
#
# Commands:
#   hubot demo-attachment - Demo Attachement

module.exports = (robot) ->
  robot.respond /demo-attachment$/i, (msg) =>
    fields = []
    fields.push
      title: "Field 1: Title"
      value: "Field 1: Value"
      short: true

    fields.push
      title: "Field 2: Title"
      value: "Field 2: Value"
      short: true

    payload = 
      message: msg.message
      content:
        text: "Attachement Demo Text"
        fallback: "Fallback Text"
        pretext: "This is Pretext"
        color: "#FF0000"
        fields: fields
        
    robot.emit 'slack-attachment', payload
```