Simple Eye Castor module

// Castor based on: http://www.thingiverse.com/thing:13042
// checked on OpenScad v2015 (it doesn't work on OpenScad v2014)
// I use PLA and G16 Hardened Carbon Steel Bearing Balls 12mm (I buy them on ebay or in Spain: http://web.maranges.com/). I don't recommend ABS or crystal marbles(too much friction).
// I use this series in order to check the best BallSize value.
// The BallSize value is at the bottom of every piece.