Added metric units and Unicode characters used in Chinese, Japanese and Korean (CJK) fonts.
Rename units.scad to metric.scad
Create new units directory and move metric.scad into it
New units files for both metric and US customary units
* Uses both variable declarations as well as functions. For example:
    3.0 * length_cm; // result is 30.0
    length_cm(3.0); // result is 30.0
* New functions metric_ruler() and inch_ruler() create 3D models of rulers
  to a specified length. For example:
    inch_ruler(36); // 36 inch ruler is a yard stick
Begin sorting files into subdirectories:
* hardware/ - all nuts, bolts, bearings and fasteners
* motor/ - all servos, steppers and other motors
* text/ - all string manipulation functions and font definitions