Split QueueService into QueueServicePoller and QueueServiceSender

* Splits up the class `QueueService` into two classes with different purposes: `QueueServicePoller` vs `QueueServiceSender`.
* The `QueueServicePoller` class is for getting messages off of a queue. The following methods from `QueueService` are moved to here: `delete()`, `poll()`.
* The `QueueServiceSender` class is for placing new messages on a queue. The following methods from `QueueService` are moved to here: `post()`, `postBatch()`.
Merge pull request #56 from iZettle/refactor-split-queueservice-to-poller-and-sender

Split QueueService into QueueServicePoller and QueueServiceSender