Upload source packages

As described [here](http://answers.ros.org/question/187924), I would like to use the normal Debian machinery to download the source packages built with this tool. I think this implementation should work, but I haven't tested it on the ROS infrastructure (obviously). It does work to run the right commands on my own machine.

See the commit message in 49fc1df1a39141627d4474beb159fa8e577ddcb6 for a description of a design choice that might be revisited or discussed further. In sum, the implementation here is a little suboptimal because of a desire to minimize required changes in downstream packages.