Implement asynchronous SMS deliveries

Note that Sidekiq has dropped support for MRI 1.9, which also led me to drop support for it in Cellular. I believe that should require a major version bump in SemVer, but it is a kind of special case. If you are already using Cellular on MRI 1.9 and update, then surely stuff will break, but if you're MRI 2.0+, then nothing will break. So I guess it's up for discussion whether or not we should bump to version 2.0, but in my opinion we should.
True. To be honest, maybe we should make Sidekiq a runtime dependency? Or, we could require the AsyncMessenger only if `Sidekiq` is defined.