Pythonz-bd

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* Remove spurious install_dir checks and use a cleaner Exception when it already exists
* Add sha256sum for every python package, and supporting code to check it before installation
* Add ability to make external installations
* Add support for PyPy 4.0.0 and 4.0.1
* Use resumable-urlretrieve to resume interrupted downloads
* Add list -a filtering by groups
* Add support for CPython 2.7.11 and 3.5.1

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* Add support for CPython 3.5.0
* Add support for PyPy 2.6.1

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* Add missing 'import sys' in locate command

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* Add support for PyPy 2.5.1 and 2.6.0
* Fix parsing versions of the form X.Y.ZZ
* Changed locate command to error out when a version is not found
* Add support for CPython 2.7.10 (6 days ago) <Chip Warden>
* Add support for Jython 2.7

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* Exit pythonz-install with non-zero status
* Fix system-wide installation
* Add support for CPython 3.4.3
* Add support for CPython 3.3.6
* Add install instructions for OS X

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* Add support for PyPy 2.5.0