Centered microswitch probe for classic Rostock 3-hole effector

This is a temporary microswitch probe that can be installed by simply clipping into place underneath one of the three M4 screws that the classic Rostock effector uses for mounting the hotend.

It has only been tested against an E3Dv5 clone mounted using my [lightweight 3-prong groove mount](http://www.thingiverse.com/thing:890466).

The purpose of this mount is to hold a microswitch centered beneath the nozzle so that Smoothieware's G32 can do its job accurately. It is also parametric, so yo