Implement heartbeat

Closes #1 
I've been thinking about this and I don't think that the `Agent` class is important in this scenario as it doesn't really bring that much additional value but does bring additional lines of code to maintain, and in the name of YAGNI and in the spirit of doing the simplest thing possible so that you have the most flexibility for when additional requirements apprear in the future rather than confining yourself already to this object, maybe it could be removed?

Instead, as there's already some information already implemented at the `TalkdeskRiemannAgent` namespace level, we could just do this type of thing there for now:

```ruby
module TalkdeskRiemannAgent
  module_function

  def heartbeat
    Instrumentation::Heartbeat.new(
      stats:  stats,
      interval: config.heartbeat_reporting_interval
    )
  end

  private
  def stats
    # I'm not sure if i'm that fond of the name "stats" rather than interactor, but it was more to see how it would look like
    @stats ||= Riemann::Stats.new(
      client: Rieman::Client.riemann_with_udp,
      app: environment.app,
      hostname: environment.hostname,
      dyno: environment.dyno
    )
  end

  def config
    @config ||= Config.new
  end

  def environment
    @environment ||= Environment.new
  end
end
```

What do you think?