Dynamic schedules do not persist after restart

Reproduce the issue raised in #269. Whenever the schedule is re-read and
reloaded, dynamic schedules disappear.
Do not clean persisted schedules

Resque#clean_schedules respects `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:

``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)
```

By default, a schedule will not be persisted.

Note: while there I've pipelined redis command for Resque#set_schedule
and Resque#remove_schedule.
Add persist config example

Update the README to show how to persist a dynamic schedule.
By default schedules are not persisted

Add a note to the README to make it clear that by default schedules are
not persisted.
Add a regression for typo in #remove_schedule

There is a typo in #remove_schedule method as noted by @potomak.
See the discussion for details:
https://github.com/resque/resque-scheduler/pull/314#discussion_r8362743
Remove schedule from persisted_schedules

When removing a schedule, remove it also from the set of persisted
schedules.
Remove commented old test set-up

Forgot to remove the old test set-up.
Merge pull request #314 from bugant/persist-dynamic-schedules

Persist dynamic schedules