Support Duration for jobs periodicity

Instead of doing
```ruby
name = 'send_emails'
config = {}
config[:class] = 'SendEmail'
config[:args] = 'POC email subject'
config[:every] = ['1h', {first_in: 5.minutes}]
config[:persist] = true
Resque.set_schedule(name, config)
```

it would be a good idea to be able to do this
```ruby
config[:every] = [1.hour, {first_in: 5.minutes}]
```