Improve performance by loading the items in a thread.

Loading each item with HTTP is very time consuming.
By loading the items in a thread the main migration thread does no longer need to
wait while loading each item.

The queue does only load a maximum amount of items (default: 10) so that the RAM
does not get filled up.
Merge pull request #3 from collective/jone-performance

Improve performance by loading the items in a thread.