Add subscriptions.coffee - PubSub notification system

This script enables Hubot to act as on-demand notifications bot. For example, you can publish
events from various systems to create a flexible monitoring solution.

Example use case:
- A room is subscribed to 'foobar' notifications using "hubot subscribe foobar"
- 'foobar' notification is published via HTTP GET or POST by other application
- Hubot announces about published 'foobar' event in all rooms subscribed to 'foobar'
- Room gets unsubscribed from event notifications using "hubot unsubscribe foobar" to stop
  receiving information about these events.