update download script to resume downloads when server supports it (fix #559)

[dirk-thomas]

@tfoote @wjwwood Please review.

[tfoote]

This whole second code branch could be avoided by using the above code block. The fetch of the md5sum file hopefully won't ever trigger a partial redownload, but the API is the same and this name poor anyway as it's not md5 specific at all.

[dirk-thomas]

This is not touched by the pull request. Simply because I did not want to change existing API.

But what avoidable "second code branch" are you referring to?

[bricerebsamen]

I tested it with my server that was making problems and it's working now...
For reference, here is the file that I use for testing:
http://cs.stanford.edu/group/driving/data/ladybug/ladybug_intrinsics.dat
it's likely to stay at this location for a long time, so feel free to use it in the future if you need to make changes.

[wjwwood]

+1