Servo wheel 4.0

This is a new version of the parameterized Servo wheel for mobile robots:

http://www.thingiverse.com/thing:9071

These are the wheels used by many printbots such as Miniskybot or Scout. The improvements of the Version 4.0 are:
* Adaptation to the 3 types of Futaba 3003 servo horns: rounded, 4-arms and 6-arms
* New drills for attaching the horns. Now it is easier to screew the horns to the wheels

The sources are stored in the Miniskybot 2 github:

https://github.com/Obijuan/Miniskybot/