[TORQUE-257] Provide support for 'at' jobs

Added suport for new way of scheduling jobs:

```ruby
# The job will be executed every 200 ms, from now, for the next 10 seconds
TorqueBox::ScheduledJob.at('SimpleJob', :every => 200, :until => Time.now + 10)

# The job will be executed for the first time in 10 seconds (current time + 10 seconds), then every 500 ms, for the next 10 seconds (current time + 20 seconds)
TorqueBox::ScheduledJob.at('SimpleJob', :at => Time.now + 10, :every => 500, :until => Time.now + 20)

# The job will be executed for the first time in 5s and repeated 3 times. The time between executions is set to 1.5s
TorqueBox::ScheduledJob.at('SimpleJob', :in => 5000, :repeat => 3, :every => 1500)
```

Added integration tests and documentation.