Fix old pkg_resources egg version normalization.

[mikekap]

Before recent releases of setuptools, version numbers in egg filenames were mangled. safe_version handles this correctly in modern setuptools and should work on older setuptools as well.

Closes #226.

[jsirois]

The function at least goes back ~forever So LGTM:

jsirois@gill ~/dev/pypa/setuptools (master) $ git log -S"def safe_version"
commit 0c9e886999bb42cf825054778b1664e41f8164b4
Author: PJ Eby <distutils-sig@python.org>
Date:   Sun Jun 5 18:59:37 2005 +0000

    Add "safe_name" and "safe_version" functions to allow sanitizing of
    distribution names and versions in arbitrary packages that might be built
    using EasyInstall.

    --HG--
    branch : setuptools
    extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041040

[jsirois]

Thanks @mikekap and sorry for the delay reviewing this. Patched into master @ 4966afa