Add local store

[dblock]

There's an interesting implementation in https://github.com/slackhq/node-slack-client/blob/master/src/client.coffee that caches users, channels, ims and groups.

        for k of data.users
          u = data.users[k]
          @users[u.id] = new User @, u

It then does a lot of bookkeeping for you listening to well-known slack events.

      when "team_join", "user_change"
        u = message.user
        @emit 'userChange', u
        @users[u.id] = new User @, u

Slack::RealTime::Client should implement similar behavior, possibly optionally.

The 2.0 branch does this even better with a clearly defined local store abstracted away.

We want to start with a local store

module Slack
  module RealTime
    module Store
      class Memory
        attr_accessor :users
        attr_accessor :channels
        attr_accessor :dms
        attr_accessor :groups
        attr_accessor :bots
        attr_accessor :teams

        def initialize
          @users = {}
          @channels = {}
          @dms = {}
          @groups = {}
          @bots = {}
          @teams = {}
        end
      end
    end
  end
end

And implement a set of handlers like https://github.com/slackhq/node-slack-client/tree/2.0.0-beta/lib/data-store/message-handlers that manipulate the store.