Whitelist external packages

* pypi_mirror/cmd/run_mirror.py: Use the --allow-external and
--allow-insecure options so that pip 1.5 and later will assent to
retrieve the netifaces, psutil, pysendfile and pytidylib packages
even though they're not hosted on PyPI. The --allow-insecure option
is aliased to a clearer --allow-unverified wording in 1.5, but the
old form is being used to avoid breaking users of 1.4.x and will be
valid at least through 1.6.x according to comments in the pip
source.

Change-Id: I43d276fd6d294f74fae6d7ef36ac38d27a53f14e