Update build_shar.sh

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.