28BYJ-48 stepper motor driver

Nice, thanks! Would you be happy to submit this when you're happy with it? To be honest nothing apart from the angle+speed makes this specific to those steppers, so it might be worth making it a more general module name?

Does this ramp the speed up and down as well? Looks like it might... Some things that might be fun:

• Ability to move an amount in a fixed time period (it's handy if you have two steppers and want to draw a 'line' with them)
  
• Make sure you can set it going to a new location in the callback function and it doesn't 'stutter' - this caught me out when I did stuff :)
  
• stop function - sometimes you might have the stepper connected to something with a limit switch, and you want to move back until the switch is pressed, then stop it and zero the counter.