Add stub support for Sidekiq::Batch

- Sidekiq::Batch is a SidekiqPro feature -- well worth it, btw.
- Use a NullObject pattern to replace the implementation
- Does nothing if you don't have Sidekiq::Pro

References:

- https://github.com/mperham/sidekiq/wiki/Batches
- https://github.com/mperham/sidekiq/issues/493
- http://devblog.avdi.org/2011/05/30/null-objects-and-falsiness/
Merge pull request #17 from kmayer/batch-support

Adds stub support for Sidekiq::Batch