Persist dynamic schedules

This PR is meant to fix the issue raised in #269: whenever the schedule is re-read and reloaded, dynamic schedules disappear.

The idea to fix this is to make Resque#clean_schedules respect `persist` config, i.e., if a schedule is set to be persisted, it won't be cleaned. In order to make a job persistent, just set its `persist` config to `true`:

    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)
    
By default, a schedule will not be persisted.