Tox-external-wheels

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- added extra support to multiwheel dependencies

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- multi wheel: added options to add dependency external wheels
- external build: option to run an external build script
- exception improvements: more human readable and descriptive errors

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- README update missing from previous version

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- added ``--external_wheels`` parameter
- renamed ``external-wheels`` testenv attribute to ``external_wheels``

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- hopefully last initial release

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- releasing is hard