Fix bug and download all remotes

This should now be fixed:
./crittertrader:176:in `join': no implicit conversion of nil into String (TypeError)

It was caused by us downloading nil hashes (critters for which upload
tokens were requested but they were not uploaded).

Also, download all server critters at once. This really accelerates
getting good critters from the server. I still only upload one because I
don't want to send too much to the server. In fact it would be good to
weed out low AD critters before sending them too, I've not done that
yet.