Fix 'motor' and 'Motor' definitions

[jmsv]

When plans were made to add stepper motor support, Motor was renamed to DC, see #48.
I thought this would be better done sooner rather than later, although people were already using Motor, for example at:

• https://medium.com/@Keithweaver_/controlling-dc-motors-using-python-with-a-raspberry-pi-40-pin-f6fa891dc3d
• https://gist.github.com/keithweaver/4e40888cab12a9eadd7a30a0d5477090

This could be fixed by creating classes inheriting from DC, printing a deprecation warning before calling DC.__init__