Use base85 instead of base64 for the bundled pip

This changes it so that instead of using base64 encoding we're using base85. This represents a savings of ~88kb for the ``get-pip.py`` script. Because older versions of Python do not have base85, this also backports base85 decoding from Python 3.4.

This does however, make it so ``inv generate.installer`` must be run on Python 3.4+