Add support for URL based volumes.

If the API looks ok, I will add it to the other API files.

thanks
--brendan
Is this the right direction for this sort of thing? Why are we building all of these custom downloads into kubelet? Wouldn't it be much more composable to provide a standard container to do these downloads, which people can just add into their pod?

It's easy for *us* to add stuff to kubelet, but it's not easy for users to do so. It is, however, easy for users to modify a container if we publish source. I mean, some user is going to see this and discover it doesn't work for .bz2 files and then they'll be stuck with no easy way to change that. If instead the autodownload logic were in a standard container, they could easily just modify it and be on their way.