Added support to Hirefire

Testing:
1- Run rack app
```Shell
thin -p 3000 start 
```

2- Access http://localhost:3000/hirefire/development/info

3- Should return number of jobs in queue for specified worker: 
  [{"name":"worker","quantity":0}]

4- Run outpost console:
```Shell
. ./env.sh
irb -r ./runner.rb
```

5- Create an envelope and post it to create a job:
```Ruby
envelope = MailCannon::Envelope.create(
  from: 'test@mailcannon.com',
  to: [{email: 'lucasmartins@railsnapraia.com', name: 'Lucas Martins'}],
  subject: 'Test',
  mail: MailCannon::Mail.new(text: 'you will see this when no HTML reader is available', html: 'this should be an HTML'))
envelope.post!
```

6-  Access http://localhost:3000/hirefire/development/info again and check if it returns the updated number of jobs:
 [{"name":"worker","quantity":1}]

Rack app staging url:
http://rdstation-mailcannon-staging.herokuapp.com/hirefire/5f7dca0c0792933e8053834f96a9de9f8782cd9e/info