Call auto-subscription to topics for subscribers that implement it

[vovimayhem]

What does this PR do?

Implements the plumbing that would enable subscribers to automatically create a subscription to a topic, if the Subscriber class implements how to do it.

This feature would be required for #17

[vovimayhem]

I'll need to add a couple tests for this change, and add the documentation to use this.

[vovimayhem]

OH! I see the documentation is actually on a Github Wiki... I'll post an example of how to implement an auto_subscribe method, so it can get included in the Wiki.

[vovimayhem]

This is a simple example of how to implement the "Auto-subscribe" method. Of course, you might implement a more complex logic if you need it:

class MyAutoSubscriber
  include Subserver::Subscriber
  subserver_options queue: 'default'

  def self.auto_subscribe
    client = Subserver::Pubsub.client
    client.topic('my-topic-name').tap do |topic|
      topic.subscription('my-topic-subscription-name') ||
      topic.subscribe('my-topic-subscription-name')
    end
  end

[vovimayhem]

@WintheDay Please take a look at this :)

[paradigm314]

@vovimayhem Sorry it took a while to look at this. It looks great!