Use a ZipFile as the get-pip.py script

Previously get-pip.py was a hacky script that included compressed base64 encoded pickled files that represented an installed pip. It extracted these files into the temporary location and then executed that unpacked pip in order to install pip and setuptools from PyPI.

This changes it so that get-pip.py is the bootstrap script and it contains inside it a base64 encoded zipped copy of pip. It will decode this base64 encoded zip file, write it to a temporary location, and then add it to the sys.path. It will then run the bootstrap function which will extract cacert.pem from the zip file, write the cacert.pem to a temporary location, and finally install pip using the copy of pip within the zip using the temporary cacert.pem to validate the TLS connection to PyPI.

This should represent no change in behavior for users of get-pip.py however it should push the trouble of the bulk of the implementation onto the Python install instead of trying to do this ourselves.

The one important behavior change for packagers is that this will no longer package an *installed* pip, but will instead package the version that has been checked out. I believe this is more obvious and what a packager would expect to happen anyways.

I have tested this on Python 2.6, 2.7, 3.2, and 3.3 on OSX and they all work correctly.