Added support for MKRZero and Created Board Test Suite

[ShawnHymel]

Arduino MKRZero pins and board definitions added for ports/atmel-samd/boards. Created a set of tests for similar ports. These tests can be run individually to ensure that pins and buses work properly when creating a new port.

[tannewt]

Hey! Thank you for this. I changed my mind on where the board test stuff should go because libraries are auto-converted to mpy on release and the readmes are easier to find. Please PR the test files to here instead: https://github.com/adafruit/Adafruit_CircuitPython_BoardTest

That should simplify this PR a bunch.

Also, please add the board name to one of the lists at the top of .travis.yml so that its auto built and auto-released.

[tannewt]

The trailing __ is needed for RST links here and above. You can see the output on GitHub by clicking View File: https://github.com/adafruit/circuitpython/blob/f8ded46ad69ebba2de5b791bde16c50c4d2b1d1b/README.rst

[ShawnHymel]

Will do. I'll cancel the PR and re-submit with the test code in the other repo. Thanks!

[dhalbert]

This error happened in the doc build in travis, I think because the link is a redirect (redirects to https://store.arduino.cc/usa/arduino-mkrzero or presumably something else that's location-dependent).

Warning, treated as error:
/home/travis/build/adafruit/circuitpython/README.rst:63:'any' reference target not found: https://store.arduino.cc/arduino-mkrzero
The command "(! var_search "${TRAVIS_TESTS-}" docs || sphinx-build -E -W -b html . _build/html)" exited with 2.

[ShawnHymel]

@dhalbert

Thanks for catching that. I'm new to Travis and RST, so this is all learning for me :)

[ShawnHymel]

Closing PR. Will submit MKRZero port separately and move test suite to different repo as per @tannewt 's request.