Refactor: Rename MessageWrapper -> PolledMessage

* Rename `MessageWrapper` -> `PolledMessage` to better show the intent of the object type.
* Remove unused properties `messageReceiptHandle` (it's sufficient to have only `messageId`) and `md5sum`.
Merge pull request #55 from iZettle/refactor-messagewrapper-to-polledmessage

Refactor: Rename MessageWrapper -> PolledMessage