Implement delivery receipts.

1) Support a "receipt" push message type.

2) Identify messages by timestamp.

3) Introduce a JobManager to handle the queue for network
   dependent jobs.
this is `true` for self texting, which leads to a delivered indicator icon on the received site too
![unbenannt](https://cloud.githubusercontent.com/assets/5326916/3787450/829b4af6-1a19-11e4-97a0-ca7557df190a.png)