S10 reduce envelope

Implemented JS MapReduce for MongoDB, the following is the reduce result for an `Envelope`:

```ruby
{       
        "posted"=>{"count"=>0.0, "targets"=>[]},
        "processed"=>{"count"=>0.0, "targets"=>[]},
        "delivered"=>{"count"=>1.0, "targets"=>["1"]},
        "open"=>{"count"=>1.0, "targets"=>["2"]},
        "click"=>{"count"=>0.0, "targets"=>[]},
        "deferred"=>{"count"=>0.0, "targets"=>[]},
        "spam_report"=>{"count"=>0.0, "targets"=>[]},
        "spam"=>{"count"=>0.0, "targets"=>[]},
        "unsubscribe"=>{"count"=>0.0, "targets"=>[]},
        "drop"=>{"count"=>0.0, "targets"=>[]},
        "bounce"=>{"count"=>1.0, "targets"=>["3"]}
}
```

The "target" is supposed to be your custom "glue" to link within your own application, like `MyApp::Contact#id`.

This data will be processed at production through the [Outpost](https://github.com/lucasmartins/mailcannon-outpost).