Pyiso8583

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- Optional proprietary header can now be parsed
using standard field settings
- Documentation improvements

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- :func:`iso8583.decode` and :func:`iso8583.decode` now return a tuple
- :func:`iso8583.decode` returns a tuple of decoded dict instance
and encoded dict instance
- :func:`iso8583.encode` returns a tuple of encoded bytes instance
and encoded dict instance
- Encoded and decoded dict instance keys are now all strings
- Specification keys are now all strings

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Initial release.