fix subprocess failing on windows due to ';' used in command_line.

Hi,

We had a contributor for whom the build failed on windows due to the unix-specific invocation of subprocess, in `publisher/build_paper.py`. The call includes a `cd out_path ; do_something` that can be changed into a `cwd` argument to `subprocess.Popen`.

Here's a patch for that, if found appropriate. Tested just on my machine for now (Linux), further tests from windows and mac welcome!

P