Adds a plugin to monitor PostgreSQL replication.

Example report:

```ruby
{
  "synced"=>2,
  "state"=>"streaming",
  "sent_location"=>"0/3010C8F8",
  "streaming"=>1,
  "archive_mode"=>1,
  "standby_receive_location"=>"0/3010C8F8",
  "replication_delay"=>139.932681
}
```