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
* units/ - all information on measurement units
Merge branch 'master' of https://github.com/McNeight/MCAD