Bug: emacspeak-bookshare doesn't wait for epub3 preparation to complete before download

[Parham Doustdar <parham90@xxxxxxxxxxx>]

Hi,

I've been thinking a lot about how to solve this, but I can't come up with an elegant way, so here goes.

Right now, when Emacspeak downloads something from bOokshare in `emacspeak-bookshare-download-internal', it just dumps whatever it is into the `target' argument.

However, when you request an epub3 book for the first time, Bookshare returns a response like this:

      <bookshare>
  <version>5.6.8</version>
  <book>
    <content-id>2095617</content-id>
    <packaging-status>SUBMITTED</packaging-status>
  </book>
</bookshare>

The response is described here: https://www.bookshare.org/cms/api-v10-download#BookPackagingStatus

It seems like the correct thing to do here is to keep trying until the endpoint returns the binary data, and not the packaging status response.

Thanks!