Reinforced J-Head Bowden mount for Prusa Mendel with pneumatic coupling

Based on EiNSTeiN_'s "Bowden mount for Prusa Mendel". Thanks for a great base!

Re-inforced design, and removal of the top piece. This design needs a pneumatic coupling to hold the PTFE tube, jut like on a Kossel (they are very reliable).

Bigger hotend hole to accommodate the old J-Head of my Mendel i2, andsome tolerance added to the hex nut holes. The code is modified quick-and-dirty.

Yet untested because mine is still printing.