Cleaning up gcodetools

Taking a stab at cleaning up some code. I use this particular extension and so I started here.

It's a doozy.

I've tried to break each of the changes into its own commit. I figured I would take a break and make sure I am heading in a productive direction.

This could be committed as is but I am hoping to do a couple more whacks with the PEP8 stick to knock some bugs loose.

Some questions:

• This uses import simpletransform. What is the deprecation path for this module?
• This has part of the code with a dependency on pypng What is best practice 3rd party packages?
• Tests would be great for this but difficult. I'll try and get some committed before I set this down.