Servo mount for auto bed-leveling z-probe

This design is based on [Servo mount for auto bed-leveling for Prusa i2 X-Carriage](http://www.thingiverse.com/thing:194012). I changed the following:

* Parametrized the SCAD source. Tweak it to make it fit your servo, X-carriage and hotend exactly.
* This servo mount can be printed longer or shorter, matching your hotend. The original is too short for E3D v5 hotend.
* Servo mounting holes are smaller, to match the smaller holes in most micro servos.

This mount will fit most X-carria