Fabtest

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* Improved workaround for a VirtualBox bug.

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* Workaround for a VirtualBox bug under Windows.

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* Packaging fixes.

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* Better exception handling (the original traceback is preserved).

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* Fabric >= 1.4 is supported (and required);
* more reliable virtualbox snapshot handling;
* ``fab`` function now wraps fabric's ``execute`` and
returns single value, not a list of values.

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* Many tweaks & bug fixes.