Added simplify_path(), slerp(), slerpn(), improved random_polygon(), minor wordsmithing of docs

[amatulic]

Changes to paths.scad:

• Added new function reduce_path() to complement subdivide_path() and resample_path(). The reduce_path() function simplifies a path by removing points that don't contribute to dominant features; useful for simplifiying coastlines and fractal paths with too many points.
• Added a newline before each assert() message.
• Minor grammar / spelling tweaks.

Changes to math.scad:

• Improved random_polygon() to accept a size range instead of a single size, to make more reasonable looking polygons. Backward compatible with original version.
• Added slerp() - spherical lerp() on a great-circle arc.
• Added slerpn() - spherical lerpn() on a great-circle arc.
• Added newlines before assert messages that I missed last time I updated this file.

[adrianVmariano]

Is this ready for merge?