Partial fix for extrusion bug

https://github.com/joostn/OpenJsCad/issues/19
fixed issues 19
Merge pull request #20 from tedbeer/gh-pages

Fixed issue 19
Added original extrude() function, renamed tedbeer's to extrudeExperimental
Merge branch 'gh-pages' of github.com:joostn/OpenJsCad into gh-pages