Crazyflie propellers and motors Protection

###Crazyflie propellers and motors protectors.
Robust but Light (less than 1 gram each).
** Easy mount-dismount:** No need to detach propellers.
Just **extrude 4 of them** and fit over existing motor plastic holders.
Possibility to put some **"foam absorbers feet"** underneath to protect motors from "empty battery" crashes.