ez_setup.py now takes a --insecure argument to bypass the secure downloaders. download_setuptools also now accepts a new keyword argument 'download_factory', enabling programmitic invocation to customize the downloader resolution. Fixes #75. Thanks to Pablo Algarvio for the report and suggestions.

Document __main__.__requires__

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Updated error message reported when `--allow-hosts` blocks a link to provide a less startling user experience. Fixes #71.

Also modernises the initial overview for pkg_resources, and helps
make it clear that it covers anything with an egg-info directory,
not just egg files.

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Powershell isn't installed by default on Windows Server 2008. Also it's possible that Powershell is present on older systems. Now use direct detection of a Powershell executable to determine viability of that technique for a downloader. Fixes #67.