add circle.yml file to control CircleCI testing - including caching tox environments (to speed up builds)
add circle.yml file to control CircleCI testing - including caching tox environments (to speed up builds)

BlackstoneEngineering/yotta,ARMmbed/yotta,BlackstoneEngineering/yotta,eyeye/yotta,autopulated/yotta,autopulated/yotta,eyeye/yotta,ARMmbed/yotta