Deal with comments in parentheses in G-Code

I have added some code to deal with comments in parentheses in G-Code. G-Code files generated by some CAM software, e.g., CamBam, use parentheses to include comments. This is specified in G-Code specification, e.g., Section 16 in http://linuxcnc.org/docs/html/gcode/overview.html . Gcode.py is modified to process and discard the comments in parentheses.