Slightly improved E3D nozzle fan shroud

Added a ring for more even airflow.  More details here: http://www.thingiverse.com/thing:210558

Currently using it on my printer, seems to help on tricky overhangs on front which airflow from original design didn't reach. Will keep evaluating for airflow evenness (might make ring slit variable-width and/or add internal ridge at back, if warranted by observations) -- if you try and have any observations, please leave comment.

Also includes parametric CAD source (redone in OpenSCAD), which should make it easier to tweak.