i3 mk3 extruder cover for i3 mk2

When putting together the mk3, I preferred the fan shroud of the mk3. So I decided to modify the i3 mk2 extruder cover to allow it to use the new fan shroud. This is a basically a merging of the Prusa i3 mk2 and mk3 extruder cover.

The i3 mk3 fan shroud is taken directly from prusa's site, I've just included it for convenience.

THIS WON'T FIT THE MK2s AS THE PINDA PROBE WILL HIT THE FAN SHROUD