convert offset var "o" to special variable "$o"

More flexible, and I think more correct based on how the variable is used.

http://en.wikibooks.org/wiki/OpenSCAD_User_Manual/The_OpenSCAD_Language#Special_variables
Drastically simplify track-standard.scad

Fewer options.

I decided that if this is supposed to represent "standard" types of track, there
didn't need to be so many customizer options.  Anyone who wants non-standard
track pieces can modify the .scad directly and re-render.

And re-render the STL files.
Render a T-track intersection.