Arc support

[Capo01]

I have been testing the arc supported branch with a pen potter on an Arduino mega and it appears to work just fine.

[Traumflug]

Thanks a lot for trying!

As Github found out already:

This branch has conflicts that must be resolved

Next to resolving conflicts, this branch needs more work:

• Wrap the new code in a #define like SUPPORT_ARC. Typically printers need no arc support, G-code generators resolve this in a more general way already.
• Add this #define to the configuration files.
• Bonus points for adding it to Configtool, too.
• Wrap arc debug code in #ifdef DEBUG.
• Remove this RESUME_PIN stuff.
• Remove this E endstop stuff (extruders have no endstop).
• Remove all the commented code.
• Fix intentation (replace tabs by spaces, indentation is 2 spaces).

In short: this code needs some love.