Proxy-toggle

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- Bugfix: fix argument name passed to gnupg constructor

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- Fix 1: Raise SystemExit with error message when RuntimeError occurs as it doesn't have errno attribute

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- Bugfix: no error reported when 'px --test' fails

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- Add missing dependency (six) to setuptools script
- Subprocess inherits parent's 'PATH' environment variable
- Add new positional command-line argument: '--clear'

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- Add new positional command-line arguments: '--version', '--test'
- Add full support for python 3.x
- Set 'all_proxy' environment variable as understood by curl

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First stable release