Better z-probe for Kossel Mini & E3D-v6

It's a new design for the Kossel Mini's z-probe. The original one with an allen wrench and a microswitch was not satisfying at all to me.

07/07/2015 Update : Known issue(s)
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- Over time, elastic bands start to shred and you can't put the probe back in place. Replace them and everything will be alright.
- As wires move with the head, they break sometimes. I used a collar around the probe body to maintain them. This workaround fixed the issue fo