Create SWT backend for Download

This allows us to push off the processing of the Download's block
to the GUI thread to avoid thread access errors.
Fix failure in SWT test

Highly suspect this isn't the right way to address this, but found that
the async_exec wasn't being called properly in the spec running. No idea why not.
Merge branch 'fix_for_download_thread_access' of git://github.com/jasonrclark/shoes4 into jason/download_fix
just spec that eval_block is an outgoing message