NinjaFlex upgrade for PrintrBot Simple 2014.1

Based on thing:337267, with following changes:

* Split into upper and lower part to simplify print and assembly.
* Upper part has been reinforced
* Lower part now is inserted into hot-end instead of being attached to extruder tension arm.


Have some hours of printing with Ninjaflex during December 2014, working OK.
Does not obstruct when printing PLA. Actually makes it a little bit easier to thread new filament.


**NB: work-in-progress**

* You may need to extend the holes in