Add importlib to install_requires on Python 2.6.

Fixes #319.
Merge pull request #320 from berkerpeksag/importlib-2.6

Add importlib to install_requires on Python 2.6.
Remove this script; no longer needed.
PyPI is now behind a CDN. The `--use-mirrors` option is deprecated.

See PEP 449 and https://mail.python.org/pipermail/distutils-sig/2013-May/020848.html
for more information.