R2 Ninjaflex extruder

So the designs that all reclaim to help extrude ninjaflex don't really work, or they have some space where the filament can escape (http://www.thingiverse.com/thing:42250), or they are adjustable with a screw (http://www.thingiverse.com/thing:136589) - this is a remix of these two (keeping the spring, enclosing the filament guide hole), plus I added a tiny blocking part under the gear wheel to avoid ninjaflex doubling up and passing under the gear - the ninjaflex is now completely supported and guided on all sides from the pinch point to the hotend entry hole - you probably still need to avoid almost any retraction at all (still to test with larger retractions), but I can now print varios parts at once, when before this caused the filament to snag after a couple of layers (heat creep + filament doubling under the wheel).