Added the following arguments
* -p - specify prefix
* -d - specify directory containing pre-downloaded dependency tarballs
Additional tweaks:
* Switched to using virtualenv to ensure that a new enough version of
pip is available. Older pip seems to have issues with downloading the
dependencies, silently failing to download certain tarballs. This
removes the hard dep on python-pip, and adds python-virtualenv as a
hard dep.
* Detect bundled zeromq in pyzmq tarball, and use it instead of trying
to fetch it. This, coupled with the new -d option, allows for the
entire stack of deps to be frozen for consistency in packaging.
