Enable ActiveJob handlers by default in rails_event_store

[paneq]

Right now we have this code ready but it is not enabled as default dispatcher:

module RailsEventStore
  class Client < RubyEventStore::Client
    def initialize(repository: RailsEventStore.event_repository,
                   event_broker: EventBroker.new,
                   page_size: PAGE_SIZE)
      capture_metadata = ->{ Thread.current[:rails_event_store] }
      super(repository: repository,
            event_broker: event_broker,
            page_size: page_size,
            metadata_proc: capture_metadata)
    end
  end
end

We could detect if active_job is enabled and in such cass pass ActiveJobDispatcher to EventBroker.

ActiveJobDispatcher or standard Dispatcher could be wrapped in instrumentation proxy #90 .

Related to #124 as well

[paneq]

Also, I believe it could be better to have dispatcher as separate dependency, instead of internal dependency of the broker.

[paneq]

After done, fix the documentation in http://railseventstore.org/docs/subscribe/ to no longer require custom config

[paneq]

rubygems/rubygems#346
rubygems/rubygems#1811