Adds dirty map management to the receiver side

Fixes #5.
Includes code from from #15 (which isn't merged at time of writing, but is getting close).

Reviewers may want to check [the updated README](https://github.com/HouseTrip/routemaster_client/blob/dirtymaps/README.md#filtering-receiver) before diving in the code.

NOTE: we may be getting to the point where some of this gem should be spun off into a separate `routemaster-receiver` gem.
Make any sense to bundle the listener and job in to one class:

```ruby
class SweepJobListener
  def self.perform
    MAP.sweep_one do |url|
      puts url
    end
  end

  def on_sweep_needed
    Resque.enqueue(self.class)
  end
end
```