Added support for excluding already installed packages, but only when requirements are not specified in a requirements.txt file. Inspired by conversations at HackIllinois and Issue 13.
2.13
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Issue 10: When launching the target subprocess, pass through the exit code.
Now renders normal output from ``pip install``.
2.12
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Allow args to ``rwt.run`` function to be passed directly.
2.11
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Issue 1: Inject a sitecustomize into the install path to work around the lack of -nspkg.pth execution. Skip the execution on Python 3.3 and later, as it will degrade the behavior in those environments as indicated in 5.
2.10
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Issue 9: Intercept the ``--help`` argument if specified rather than passing that to pip install.
2.9
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Issue 8: Add a console entrypoint, so one can invoke simply ``rwt``.