Many fixes in parsing, rendering and new features

Hey @hamiltonkibbe, great work you are doing on gerber-tools. From your last commits seems like we are heading on the same direction now: DRC.

That's my main goal now, original one was parsing and rendering. Would be great if we could merge as soon as possible so we don't differentiate too much.

Could you take a look into some of my latest fixes and maybe merge it and then I could merge your branch back?

Cheers,