Compatibility issues with Repetier (and other) arduino based firmwares

- Repeat G1 where needed, within brackets in this example
G1 X0 Y0 Z0
[G1] X0 Y0 Z0
- Put some initilization commands (G20, G90 etc) on separate lines
- Hint in code where to make code compatible with firmwares that doesn't
support ARC (G2/G3) - not needed any more by me since I switched to
Repetier.
- Used semicolon before comments which is the only supported comment
style in repetier