Adafruit Motor Shield V2 support

In addition to Adafruit support, address these issues:

* Tests were not checking to make sure that pins were being set in the
right order #433
* Fixes the unrelated start(), stop(), start() issue brought up by
@meshy in the #433 discussion thread
* Broke out resume, setPin and setPWM to wrapper functions for
simplicity
v0.8.10

- Motor class upgrade: Adafruit Motor Shield V2 support