add PushDownloaderJob

I tried to add a job to download the push attachments to fix #1264. The job is added to the new JobManager in `SendReceiveService` and should start the `PushDownloader`. 

Unfortunately it's not working as expected:
If there is no network connection when the job is added, its delayed and the download starts as soon as the connection is established again (like it should). But if the download fails because the connection is lost during the download the `IOException` is thrown and the job seems to be deferred, but its not started again when the network connection comes back.

I thought perhaps @mcginty has an idea why the job is not started again? What did I forgot?