fetch_libzmq always downloads fresh libzmq

Forces a fresh dowload of libzmq, which allows `./configure` to run.

Should be useful on platforms where the shipped platform.hpp does not work, but bundled libzmq is still necessary.

To use it:

    pip install pyzmq --global-option="fetch_libzmq"

Of course, this won't be available until the next release.

closes #333

I also changed all of the print() methods to info(), to avoid weird out-of-order issues due to writing to both stdout and stderr.