sorted digital_pin output by location and documented digital pin repr

[gfth4x07]

No description provided.

[Vido]

Gabriel,

Thanks for you contribution.

Please, follow the PEP8 coding style:
You can check it using flake8 --ignore=E501 pingo

pingo/board.py:270:1: E101 indentation contains mixed spaces and tabs
pingo/board.py:270:1: W191 indentation contains tabs
pingo/board.py:270:1: W293 blank line contains whitespace
pingo/board.py:271:1: W191 indentation contains tabs
pingo/board.py:272:1: W191 indentation contains tabs
pingo/board.py:273:1: W191 indentation contains tabs
pingo/board.py:274:1: W191 indentation contains tabs
pingo/board.py:274:1: W293 blank line contains whitespace
pingo/board.py:275:1: E101 indentation contains mixed spaces and tabs

[coveralls]

Changes Unknown when pulling a0848f0 on gfth4x07:master into * on pingo-io:master*.